A coworker has a 94 GMC 6.5 that makes a peculiar thumping or banging noise near the fan/box assembly under the dash.

What's weird is that it only does it for about 10 secs after the key is turned on and then also does it for about 10 secs after the key is turned off.

After the banging episode ends (key on), all the ac/heater control functions work fine, all fan speeds work, and no strange noises - just during power up and down.

Maybe one of the control doors is trying to open or close, or the fan is running on its own against a closed door? Switching the fan to off before shut down makes no difference.

Any thoughts? Weird.

Thanks.

Joe.