Weird chirping noise

[Kevdog]

I'm experiencing a very odd phenomenon in my '98 ML320. About a year ago, I started getting a high-pitched squealing noise very occasionally at start-up (usually in cold weather). It would squeal for about 15-30 seconds and then go away. I always assumed it had something to do with the radio since it sounded like high frequency interference or something, but it was so rare that I paid little attention to it.

About a month ago, it started happening more frequently, and not just during start-up – it will come and go during normal operation of the vehicle. The best way to describe it is the sound of a chirping bird – it flutters and chirps for a few seconds and then goes away and comes back. It sounds like it is coming from behind the dash on the driver's side, but it's a tough sound to isolate. I just had the harmonic balancer replaced, so I don't think that's it.

Could it be the e-box fan, or something else? Has anyone else experienced this? It's going to drive me nuts if it gets any worse.