Knocking under dash

I have a 1992 Chevy 3500 PU with 111k miles, Lately when I start it or when I turn it off it sounds like someone is knocking under the glove compartment.It makes no diffeence weather or not the a/c or heat has been running, I had the blower motor checked and they said that all is well. Anybody have an idea as to what this could be?

I recall someone having this complaint and it turned out to be some air in the heater core.

Exactly what I was thinking, air bubbles trapped in the heater core. Try burping the system, open the radiator cap (with the engine cold) turn on the heat, start the engine, look for air bubbles.

I would agree with the air being trapped except I had this issue with my Nissan Altima and it sounds more like rain falling in your car or more exactly a novelty rain stick. It might sound different in your car though. I agree more with Tom when he mentioned it sounding like a relay. I read a post someplace with someone that had a similar problem with a Sentra and it turned out to be the Dashboard and Breaklight Lamp relays that are clicking which in turn pointed to an alternator problem. Basically when the key is turned and has the lamps on the relays are clicking and it really could be due to a battery going bad if it happens with the car not on or perhaps still the alternator since it is part of the circuit even if it suppose to be passive when the car is not on. At this point it gets beyond my logic and out of my depth. I would look at that though and why not burp the system to make sure since it’s easy to do yourself and free. In fact go over to a local Auto Zone if you have those around there and they will hook a meter to your system for free and at least let you know if your battery is good and getting the proper output from your alternator. They did it free for me when my battery died seemingly prematurely but maybe I just have nice Auto Zone people around here. Good Luck!