1993 Chevrolet Truck

Noises problem
1993 Chevy Truck V8 Four Wheel Drive Automatic 235000 miles

about 15 knocks regularly spaced sounds. Abit faster than one a second. When I turn key on and when I shut down truck. It goes away the heater seems hot. I dont smell any antifreeze, or see any fogging on windows. Its hard to tell of a leak everything was armoralled but there looks like someone added a towel under the rug (not sure)but I dont see leaking now.I was told sound is common. Lol. It doesnt sount mecanical but more like?Clogged heater core?But if I can confirm it. Isnt. Leaking is there a problem. Or future problem
November 8, 2010.

Thank you for the donation.

What happens if you turn off the blower motor? Does the noise still exist under the same circumstances?

Will the noise change of you change the speed of the blower motor?

Update I will check the blower but if I remember right no differance as I left it on once. And I was looking to buy a plow, the man owned 2 chevys and was a mechanic. His was a 90 2500 but almost the same, he listened to the noise then said it wasnt the heater(clogged )like I thought. I tried the fan for him. And its usually the same number of knocks give or take. With no change in sound.I turn the key on weather I start it or not and when I shut off. He said maybe like a fuel relay.(I thought was neer fuel tank )but it starts every time seems to get fuel fine. He checked as I did for a leak or smell from motor there seemed to be none.I asked about someone putting a towell under the rug but he said it was normal look. So it SEEMS not to be heater or blower. But a key actuated thing. Lol. Help

Nov 9, 2010.
I'm leaning toward a fuel pump.

When the key is in the start position it energized the pump. When turned off it de-energizes the pump.

If I remember right there is a fuel pump dampner on those that didn't always work out with the aftermarket pumps.I think there was a TSB on it.

Next time you start her, have someone back by the fuel tank listening. Put a funnel in the tank if you want to amplify.

Am I listening for a similar sound with same pattern?I ask because im 100% positive noise comes from behind glove box. Ill listen to see if I can hear a pumping noise with same pattern I think your asking. With my limited mech skills it sounds like its possible so the nxt question would be, it starts and runs fine so whats behind the glove box to be fixed, the knocking is rather loud. Id like to read the fuel pump recall notice

Nov 9, 2010.
Okay, definitley not the fuel pump dampner. Whole diff situation. Now I see where we are.

It should be the a bad actuator motor for the recirc door. It's designed to go into recirc mode after Key-Off, Located behind the glovebox. A gear inside strips out and that is what you hear. There is no easy way that I know to change it, two small bolts, it is very tough to get but it can be done.

Advice on pulling a fuse or un hooking a wire. Im not shy on sawz all the glove box backing to not change it but kill it or un hook it. It seems a useless part.I will try to do research on the part and what it does. So its good news not the leaking heater or the fuel pump. Thank you.I need to fing a chiltons for exact location to exterminate this part lol. Any suggestions would be great.

Nov 10, 2010.
The Recirculation Valve is normally in the position that allows outside air to enter the system. When RECIRC is selected, the Motor changes to the RECIRC Air position. If RECIRC is selected again or defrost is selected, the Controller will provide voltage to position the Motor back to the outside air position. Since there is no potentiometer on the Recirculation Valve, the Controller drives the motor for 22 seconds to ensure the proper position is attained.

The mode actuator is located on the air distribution case between the air distributor and and blower assembly. Position the actuator properly to the door shaft before installation.