knocking noise from dash!

Have a 93 c1500, when starting truck or shutting off, I have a knocking sound coming from dash. Does it for about 20 seconds then stops. Seems to be coming from the passenger side. Any ideas?

sounds like the fuel pump noise i always hear from mine.

recirculation door actuator under right side of dash stripped internally, need to replace actuator for recirculation door

recirculation door actuator under right side of dash stripped internally, need to replace actuator for recirculation door Thanks for the tip...will try that and hopefully it will cure it.

How difficult to replace? What is the cost?

I finally replaced this. It was the recirculate door motor. It took about 30 mins to get out and 15 mins to get back in. Not too difficult, but easier to unhook and reach top bolt (I broke the mount for the top one, but it still works with only one bolt) if you remove glove box. Part was $85 from dealer.

I heard a noise like that in a cust vehicle @ work the other day...had no idea what it could even possibly be but its cool that I know now.

I was also told by a GM mechanic that it is common for the Defroster actuator to have a similar problem and sound. That is on the driver's side or middle versus AC recirc which is on passenger side.