Depending on the type pickups you are putting in can often cause more unwanted noise. If you are using single coil pickups wound very hot or to a higher DC resistance, you will pick up more 60 cycle hum because of the increased turns and output. At times too, a player will get all the modifications done because he has a new amp use. You can get more unwanted 60 cycle hum from the larger transformers that were previously used in the smaller amp. Also when the pickups were put in, there is a chance the single lead wires are longer and can pick up extra unwanted noise. Make sure they are cut to minimum length and are cut nice and neat. The majority of pickup replacements are done to give the player an increased output to gain more sustain and overdrive. When you use stronger magnets, hotter coils with more turns, larger amps with larger output transformers you can get extra unwanted noise than what you are used to hearing. Older instruments have sometimes have weaker magnets and pickups that are not has hot as the newer pickup models being made. I like the sound you get from weaker magnets because of the warmer smoother tone.