Usually it's 60-cycle hum, which is common on strat pickups. There are a lot of different types of noiseless strat pickups though which get rid of that, but preserve the tone.
There could be a grounding issue on your guitar though. A continuity tester might help you out there. Crackling (if you've experienced that) is usually oxidation either on your input jack or in your pots.
I want to take a second and recommend an awesome dvd (you can also get it in vhs) by Tracy Longo.
http://www.metalmethod.com/guitar-repair.htm
He does guitar setups for a lot of the big acts out in California. This video gave me the confidence to fully set up my instruments by myself. He basically goes over every possible thing relating to guitar setups (electric), but the same principles apply to acoustic guitars as well.