chirp/squeak noise just off idle when cold

[tadc]

Greetings Yarisheads... my DW has an 07 3-door stripper automatic with about 54k miles, great car, zero problems.

Recently, with the onset of cold weather, we've noticed a strange noise that seems to be coming from the engine.

It's a bit hard to explain, but I'll try... it doesn't do it at idle, but when the car is cold and you're just starting out with very light throttle, you hear a kind of oscillating chirpy-squeak kind of noise. The most similar sound I can think of is that squeal that some cars power steering makes when you crank the wheel all the way to the stop.. just quieter.

It seems to be RPM-related, as it goes away when the engine speed increases or decreases - it only happens just off idle. It seems like it gradually changes to a slightly higher RPM as the car warms up, and eventually goes away entirely.

It kinda sounds like a belt slipping a bit, but the belt seems to be fine and it only does it under those specific conditions, which doesn't seem like a belt (which I would expect to slip more under heavy throttle starting off or when it's wet out).

So.. any thoughts? I've read about common water pump and alternator failures, but so far they both seem fine...

Thanks for any input.