I have a strange engine noise that is dependent to RPM's, but only occurs when I turn the heater blower on. The sound comes from the engine compartment not under the dash. As soon as I turn the heater on it sounds like I am over revving the engine. Anyone have any ideas?

 

Is the engine RPM increasing when this happens?

Find a length of hose to use as a stethoscope. Then track down the location of the noise.

There is nothing in then engine compartment related to the the heater blower. The blower is located under the passengers side of the dash behind and below the glove box.

 

The noise only happens when I turn the fan on. The sound increases when I press the accelerator down, but if I turn the fan off the sound goes away.

 

till something fails . these types of noises hard to narrow down

could be a voltage drop off at the battery or alternator

air con compressor clutch /blower motor/ fan speed relay ?