Weird Sound when accelerating with heater on?

This is my 1st winter with my '00 GT, & I noticed a sound that can only be heard when accelerating with the heater on. The sound gets progressively louder the more I depress the accelerator. When I turn the heat off...no more sound. Anyone else experience this? I'm wondering which heater component(s) are causing this.

can you describe the sound (whistle, hum, squeal, etc...)?

Any other issues with the HVAC system, (ex. doesn't switch modes, low heat output, loss of blower speeds)?

I would describe the sound as a hum, it's not too loud, just enough to hear. As for my HVAC system, I do feel that my upper vents blow on the weak side. The modes all switch as they should & blower speed sounds & feels right (except for the upper vents). I read a thread stating there's a hose running from the firewall that could cause low heat output if clogged etc. I haven't checked for this yet, however.

I would check the A/C compressor. When the pulley bearing starts to go bad, the compressor will make noise when it's not engaged. When engaged, the clutch pulls the pulley in and the bearing is spinning at the same speed as the compressor input shaft so the noise goes away. The compressor will be engaged when the HVAC system is in A/C or defrost (it uses the evap core as a dehumidifier to speed defrosting) so the noise should change when the control is set to either of these settings.

I just realized that this weird sound only happens when the dial is set to defrost the front windshield, or front windshield & floor. These are the two modes I use most frequently. The sound is not heard when using the heat on any of the other setting.

When turning the dial from one of the two defrost modes, a sound like air escaping a tire is briefly heard. Not sure if this is relevant?

That noise that you are hearing when changing modes is normal. I have the same noise when changing between the modes.

Since the noise only happens on defrost, and the A/C compressor switches on for dehumidifying the air in defrost mode, I stand behind what I suggested in post #4.

I'll check the a/c compressor soon as I get a chance. Thanks again.