There is fret buzz on the open string g and lower e and also on some other frets. However, I realised that when I lay the guitar flat down horizontally and play the same note, it doesn't buzz. What seem to be the problem?
Quite simply, your neck needs a slight adjustment.
The neck is pulled forwards by the strings and inside the neck is an adjustable metal bar (called a Truss Rod) that pulls the neck backwards to counter the pull of the strings keeping the neck straight.
If you have recently changed to a lighter set of strings or there has been a spell of very dry weather, then the truss rod may be pulling to much on the neck creating a slight back bow.
This means that on certain strings and at certain points on the fingerboard you can get excessive fret buzz.
Here is a link to a page on our website that shows you how to adjust the truss rod to alleviate the fret buzz - http://www.guitarbitz.com/trussrod-adjustment-i50
When you lie the guitar down, gravity is acting on the guitar and pressure on the headstock (where the tuners are) is giving relief to the neck and so eliminating the back bow.
Hope this helps
The only logical explanation I can think of for the buzzing not being present when the guitar is lying flat down horizontally is that the way you pluck the strings is different than when you are holding it in normal playing position. The oscillation pattern of the strings as they vibrate will change when plucked or picked from a different angle.
It's quite possible that a simple truss rod adjustment might solve your buzzing issue.
However there are several other factors which could cause buzzing that may have nothing to do with the truss rod. For example, if the slots in your nut (at base of headstock) have become worn and are too deep, or if you have switched to a lighter gauge string, certain strings could be too close to the frets. Alternatively, if you have an older acoustic guitar with a plastic saddle (the opposite end of the scale from the nut), the strings can cut grooves in the saddle resulting in some strings too close to the frets. Also, you could have a fret that has worked itself loose and is higher than the others and may need to be re-set or filed down or both. If you have a guitar with an adjustable bridge, and you can raise the height of the bridge, that may very well solve your buzzing issue.
Bottom line is, you can take a trial and error approach - but you might be well advised to seek the advice of a professional guitar technician or luthier who can identify the specific problem as it relates to your guitar, and make any needed (but not unnecessary) adjustments.
One thing you can do that will help you assess the trouble, is to take a sharpie marker, and play each string one at a time. If the string buzzes in a particular fret on a spot, mark the top of the fret with the sharpie. So do this for each string at each fret position. Once you're done look at the marks and the placement of where they're at on the length of the board.
Is it concentrated too much at the first few positions of the frets and fingerboard? Is it closer towards the bottom end of the fingerboard, or in the middle? This can help give you a nice visual of whether or not its your neck that is truly off or if its the frets itself. If you have a patchwork of marks on your frets in any of the three main positions on the neck, then you know you need to either give relief or take relief away from the neck.
If you have just a few marked spots on your frets that are completely random.. than you just have some high spots on your frets that need to be taken down and leveled. You could do this yourself with a small file and some varying grits of sandpaper. Simply remove a small amount of material from your frets high spot, the marked portion. And then smooth it out with the sandpapers, and finally polish the fret with some 0000 steel wool.
That should take care of your buzzing. If its a matter of your marks being in one position on your fingerboard, and you've already adjusted your truss rod and still not getting rid of the buzz.. then its an issue with the neck itself and warping possibly. Say you have added tons of relief to the neck to get rid of the up bow, but you still experience buzzing at the first position of the fingerboard, you could try shimming the neck pocket at the front of the neck pocket.
Just remove your neck and place a piece of card stock or something similar, and place it in the neck pocket where the first two screws fit into the body and neck. Then screw it back in place and tune up to pitch. That could help eliminate the buzz. Just some ideas to think about too. And don't be afraid to adjust that truss rod! Just think of it as a clock, you only need to turn it a small increment at a time, just like a hand on a clock moves. Whether you're going righty tighty, or lefty loosey, think of it as a clock and your hand only moves a slight turn or click. You don't need to turn the allen wrench a full turn or anything.
Some aids for diagnosis:
Before you go having the neck adjusted, check the nut. That's the bridge at the tuning peg end of the neck.
If you're having string buzz when playing open strings, it could be that the neck is overly bent back (towards "behind you" if you were wearing it), as other have stated. That means there may be hump in the middle of the neck against which the string buzzes. If that's happening, then it's the neck that needs straightening.
You can test this - using the G string (iddle of neck, not wound string)( fret it on the 1st fret and also on a high one, maybe 15th or so.
Assuming the string is straight (fair assumption!) you're using ist as a ruler against whcih to chekc the fretboard straightness. A bit of wavering about the middle is ok, but more than 0.25mm or so and there's cause for concern - although it's not an exact science.
It could be that the gooves in the nut are too low. This would mean when played open, the sting is too near the fretboard ('action' is too low) and buzzing results.
If this is combined with the bridge being set too low (same thing at other end) then you'll get fret buzz across the guitar, regardless of how straight the neck is.
What's "too low" ? I recently bought an electric guitar where the action was seet at 1.5mm on the 12th fret. For me that's a bit high- plus bear in mind the string flaps about less at the nut, so I'm guessing your string ought to be circa 1.5mm at 12th fret, and lots less at the nut but high enough not to buzz.
You can experiment with heightening the nut by slipping thin bits of paper under the strings. It dulls the sound but gives you an idea of what's up. The wrappers of a popular string manufacturer are about the right thickness for this.
Although far from exact, a quick and simple method is to "sight" down the neck. That is, hold the guitar outstretched from your face (with the bridge end closest to your face), and look along the edges of the fretboard (one at a time). What do you see?
You should normally see a wee bit of concave curvature -- that's what your truss rod adjusts. If you see twist, that may be warping.
The bottom line is that this is an easy method to get a general idea of the neck setup. There are many other techniques and tools to measure related things more precisely.