My '99 V6 just started making a noise like a cordless drill, and at low speeds. Take off, accelerate, and it does this 'bzzzzt' sound, varies in duration, and doesn't do it over 30 or so mph. At first I thought it was the fuel pump, the sound is like a bad electric motor, almost sounds like a cordless drill. I think the truck has a vacuum leak as well, because for the past few weeks, the heater switches to defrost, but only on hard acceleration (downshift).
The truck has had almost no engine work since new, only air filter, brakes, oil changes. The vacuum lines have not been disturbed at all.
I'm confused as to what to look for, any advice for this problem appreciated.
BTW, the truck runs fine. It did have error code PO171 (I think) months ago, the 'lean on O2 sensor bank 2 code', but I reset the OBD, and cleared the code. CEL has not been on for months.

 

Check the PCV valve as well as the hose that runs from it to the back of the intake.