When I have the ignition on engine off I can hear a fan spinning up and changing speed from what sounds like inside the cab, is it the PCM or something? Anyone else notice this? I did some searches, but just got stuff on the cooling fans.

 

The PCM is at the front of the engine bay. Is it your cabin ventilation fan?

 

Possibly, the strange thing is that it seems to turn on with the HVAC system on or off. It sounds like a computer fan, not so much a blower fan, and it sounds like it is coming from under my seat.

 

You could be hearing the fuel pump. Really hard to nail it down from a forum post :/

 

Just to answer the question, I eventually figured out the PO installed the amp for the subs under the driver seat. It is actively cooled (the fan noise). I thought the amp was in the sub box until I went hunting for it.

 

If you turn the key to run but don't start the engine the fuel pump runs to prime the fuel system. You may be hearing that.

 

Glad you were able to find the issue. Aftermarket stereo stuff can cause quite the headache in many ways.