Heater Problem

[Cubdriver]

Just yesterday the heater on my 1993 Siverado short bed, started making a hellacious banging noise. Sounds about like if someone was banging on the hood with a ball bat.

I think it is a heater damper banging open & closed, at least the noise is coming from the heater box.

It is definatly not an overheating problem. It only does this for a minute or so just after I start it up or shut it down.

Im thinking it is some sort of a vacuum, diaphram or perhaps a control problem. Near as I can tell, everything still works. I have used my air conditioner a few times this summer but it doesn't seam to matter if it is air or heat.

I bought this truck new and it has never done this before. Any advice would be appreciated.