1994 GMC truck thumping noise under dash

My 1994 GMC 1/2 ton truck started making a loud thumping noise under the right side of the dash. It happens everytime I turn on or off the key. The thumping will last about 20 seconds and then stop. It sounds
like the heating fan is hitting something in the heating cowl area. Could there be a relay or vacuum device that is supposed to open or close a door that isn’t working properly? any ideas? thanks
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My 94' chevy silverado does the same thing and has been for years. I think it may be a relay, but not sure. The brief tapping sound hasn't seem to affect anything else, so I havn't worried about it. I would like to know how fix it as well.
I'm still in need of a grill for this truck. See my other post.
thanks
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On Feb 6, 11:20 am, snipped-for-privacy@yahoo.com wrote:

Have you tried www.car-parts.com for your grill?
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I took a look at www.car-parts.com after your reply and I have located some possibilities.
Thanks
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Glad to have been able to point you in a direction.

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