So last fall I started having a subtle high-pitched squeal when I had the a/c fan on low. Now that it's warm out and had some free time I pulled out my blower fan and cleaned it (only has 50k logged and wasn't very dirty). Put it back in and put the fan on low. No noise.

Then I remembered I had the heat on when the noise was present. Turned the heat up to hot with the fan on low....squeal is still there. Then popped the hood and the noise seems to be coming from the back center of the engine bay but could be in the dash for all I know...just easier to hear when the hood is open. Any advice?

 

It might be that your heater core is obstructed a little and making the coolant whistle as it passes through. I would suggest taking the inlet and outet hoses off (on the fire wall towards the passenger side) and stick a garden hose in the outlet, then in the inlet a few times and see if that helps. Here's a crappy pic of where the hoses are: