KNOCKING SOUND AT START& SUTDOWN

[scratch]

I'VE A 94 HD 6.5 TURBO 2500 4WD.1WHEN I START THERE IS A KNOCKING NOISE AT FAST RATE COMMING FROM PASS.SIDE UNDER DASH.AFTER ABOUT A MINUTE IT STOPS.IT HAPPENS AGAIN WHEN I SHUT DOWN.JUST TURNING THE KEY ON DOSN'T CAUSE IT TO HAPPEN.I HAD THIS PROBLEM BEFORE FOR TWO WEEKS AND THEN IT STOPPED.ANY HELP OUT THERE? THANKS RICH

[CleviteKid]

Can you have someone else start your engine while you stand beside the right front tire with the hood up, listening for the fast knocking sound? It would help to know if the noise is from the engine or really from something under the dash.

Many notebook computers have a built-in microphone, or you can get one cheap at RadioShack so you could record a wave file for us to listen to.

Dr. Lee :confused:

[farmerherb]

I have a friend who had a knocking noise under the dash. It was the electric vent switch opening and closing. We played with the heater control switch and found if you hit the recirculate button on the A/C heater control the noise stopped.

[rjwest]

Theres something in the blower, when it comes up to speed it gets blown out of way, when shut down
it falls back and hits blower cage.

If it sounds like a playing card on a bicycle spoke. thats it.

Had a bussines card in mine, no idea how it got there.