Effects such as chorus, delay and reverb should be placed after gain; otherwise, your signal will be noisy.
Hum or buzz is almost always caused by improper grounding, a missing ground connection, or ground loop. Ideally, your rig should connect to earth ground (the power outlet's 3rd prong) at one and only one point.
- For multiple grounds, try a ground isolator on all but the first ground plug.
- An ungrounded rig may need to be modified to add a single ground connection. Note that most pedal power supplies (even with 3-prong plugs) isolate the pedals from ground.
If you have hum or buzz when using two amps, you have a ground loop. Refer to this article about ground loops.
A true-bypass pedal that "pops" when switched is caused by differences in ground voltage between the pedal and amp (or whatever it eventually connects to). This can be solved by using a buffer at the end of the pedal chain or (if you are anti-buffer) by using a transformer isolator.