How do I troubleshoot noisy power lines for power-line networking?
You could start by turning off circuits at the service panel one at a time until the noise level drops.  Once you have it isolated to a particular circuit, turn off or unplug devices until you find what's causing it.  The usual suspects for electrical noise in houses would be motors -- fridges, washers, etc. -- and fluorescent lights generally the 4' or 8' long tubes with magnetic ballasts, less so with CFLs (see Brian's comment below), so you should start with those circuits first.

If you can't isolate the problem that way, and if you can power your networking equipment from batteries (laptops, UPS, etc.) you could turn off the main breaker to the house and see if the problem is inherent to the wiring or environment.