electrical issue - differential diagnoses?

[2000ZTS]

Ok, so this is a long shot... I may not have enough info yet,

(I'd posted this in Zetec tuning... moved it myself and deleted other post)

2000 Focus ZTS w/factory sound... recently started making feedback/ground loop sort of noise from speakers - it is louder sometimes (not sure what makes it louder/ go away)... has been happening for last couple of weeks since it got back from putting in a new clutch.

Today on way to work with A/C on, lights on, radio on, driving about 75mph on highway... with very low/ barely audible whine from speakers... the battery light came on!

First I'm thinking the battery is developing a bad cell? Then, still in gear but not applying accelerator pedal, going down a big-ish hill... the light goes off! huh? Decide maybe i should shut off lights, and see if the battery light stays off (maybe it's charging now?)... nope... in a few minutes light comes back on... totally mystified.

When I stop, I check the terminals - they're tight.

The car does have underdrive pully set on it... but it's had that for years now... with no such problems/ symptoms.

*NEW* OK, just realized that it really only makes noise when turn signals, brakes are applied... and that the A/C and especially the lights make it louder.

when the turn signal makes the noise... it does something like: "mmmmMeeeHmmmmMeeeHmmmmMeeeHmmmmMeeeHmmmmMeeeH" (the mmmm is when the light is not on, the higher pitch happens when the turn signal lights up).

The battery is about a year old - AC Delco 72 month warranty batt (40R6YR)

Thoughts? Any thing you can think of that might be causing this? Should I worry about the car not being able to start? How/where or should I test the battery? Other things to look for/test?

What sort of "system" problems might be causing this?

Thanks for any ideas - anyone?