Noise on audio output when using the power adapter

I'm a musician and use my mid-2007 MBP 15" in my live rig. When I use the power adapter, I hear a buzz, sometimes accompanied by a slightly louder buzz that cycles on & off about 1x/second. There are a few weird things going on here:
1. The buzz does not happen all the time. That leads me to think it may be noise in the AC line that the adapter is plugged into.
2. The noise is heard when I use both the built-in audio output AND an external USB audio interface.
In any case, when I disconnect the power adapter and run my MBP on battery, the noise goes away completely. I can probably do this on most gigs, but I'm a little nervous since I run a FW drive, an external USB audio interface, and my midi keyboard (powered by USB) so I am putting a little heavier than usual load on the MBP's battery.
As I said, this may be a noisy AC line. Another guess is that it's the PMU inducing noise into the audio circuitry. I'm hoping a simple AC line filter can help me but I thought I would ask here first in case anyone else has dealt with this. TIA for any help!

I had a similar problem and fixed it with a $5 universal travel adapter. Yours might be different but here's my situation. Try it for little cost and see if it works.
I'm also a muso with an iMac running external KRK speakers via USB Mbox2. Previously I had a Macbook Pro and the same audio problem. I've been putting up with a very low volume, high pitch noise until now. I simply plugged the Mac power cord into a universal travel adapter which then goes into the wall. For Aussie users there is an adapter on ebay that will allow a 3 pin Aussie plug to go into female side and has a two pin Aussie male on the other. For US users you can probably get the same although there is a 59c product that will do the trick called a ground lift adapter. Applecare actually agreed this was the best fix and confirmed there would be no problems with power supply or Mac performance. What a gem ! God knows I've looked into all kinds of alternatives that could have cost hundreds instead of $5.

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