1993 Chevy Truck noise from heater?

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

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

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.