I have a similair problem but only when the fan is on. the RPMs control the voltage somewhat which might be the problem of it getting louder off and on. I know my problem is part of the hvac system behind the glove box on the passenger side. Mine started doing it about a week maybe 2 weeks ago.

95 SES Sold 99 SE Sport Sold 99 SVT T-Red Tan Interior. K&N, Magnecors, 19' Theorys, And some audio stuff.