I just got a new trumpet. When I press down on any of the three valves (without blowing through the trumpet) I can hear a small 'pop' kind of noise. It's not coming from the springs or anything, because it has a pitch. I was wondering if this is normal or not. Thanks!
It's not a defect. Imagine if you were to submerge a ring in soapy water and then pull it out you'd often get a sheet of bubble membrane across the ring. That's pretty much what's happening inside your trumpet valve. When you depress a valve, a bubble membrane made of valve oil can form across the airway that's opening, and when it pops (the oil film is even less stable than soapy water) that's the sound you hear. It doesn't happen when you're playing because the airstream though the instrument means the membranes don't even start to form.
Pistons should be pretty close to silent. If you have a Yamaha or similar trumpet that has a tighter tolerance than other pistons, you can try using a thinner piston oil. If you have an older ("Classic") trumpet, the tolerance will be greater and you'll need to use a thicker oil. I, and my repair techs, recommend hetman oils for tight tolerance horns. I find the #1 to work very well with my Yamaha euphonium where otherwise I get a similar sound.
Additionally, I've had recommended to me 5 drops of oil on each piston per day or 5 hours of playing.
Some brands of tuning slide grease will cause the pistons to have sluggish action and noise (from friction) when they come into contact with certain piston oils piston oils. Since we can't hear the sound, it is neither possible to tell if the noise is good or bad nor the source. It could be an indication that your tuning slides are sealing VERY well (which is good) or it could be slightly leaky (which isn't necessarily bad, but needs a thicker grease).
If you decide to use hetman oils, they also make tuning slide grease which is good for trumpets because the tuning slide grease dissolves in the hetman oil and vice versa, so they don't interfere with each other. For tighter tolerances, a lighter grease will keep the tuning slides moving smoothly and from corroding. For looser tolerances, a heavier grease will keep the instrument from leaking air while also maintaining a smooth slide and keeping corrosion away.