2002 GMC Sierra noise

My 2002 GMC Sierra, with 80K miles, has a noise which seems to be under the left front area of the truck. The noise only occurs after starting the truck and driving. The noise occurs in about 30-45 seconds after driving forward. The noise is a non-metallic sound, about 5-6 low tone raps in rapid sequence, not too loud, but audible in the truck to the driver and passanger. It seems to be related to the ignition and initial startup. It recurs after each initial startup and driving forward, but only in that initial startup and driving, not at a stop or coming out of park without turning the truck off.

Interesting problem. First, let’s just make sure we’re using the same point of reference. By left, you mean the driver’s side of the vehicle, correct?

The noise seems to be forward of the cab, in the engine bay?

The vehicle has to be in motion for the noise to occur? What if you stop just prior to the normal time when the noise would occur, does it still make the noise or not? If not, what about continuing forward again, will it make the noise or not?

The rapping is always the same, consistent timing, it doesn’t change in frequency based on either engine or vehicle speed?

Yes, driver’s side of the vehicle.
The noise is forward of the cab, but could be under the vehicle, as well, could be on the wheel/axel.
Yes, in motion to occur. Don’t know about stopping before the noise, then continuing. Did not try that.
The rapping seems consistent, no change in frequency.

Hmmm, a real puzzler! If it’s not related to engine or vehicle speed, we can rule out a bunch of possibilities. Two, top of the head, suspects for me would be; the evap solenoid or perhaps the ABS self test. The evap solenoid should open at the point where the vehicle starts moving and can consume the gas vapors from the charcoal canister. This operation varies between implementations and so might not be correct for your particular vehicle. Most ABS systems do a quick self check when the vehicle begins moving. Often this involves a periodic application of the front end of the system but does not apply the actual brakes. Have to think on this some more…