91chev p/u noise under passenger dash

Need help on this one! My 91 chevy 2500 has a fairly loud tapping sound under the passenger side dash, i used a mechanic’s steth-a-scope & think i narrowed it dwn to left of the blower motor on the inside of the firewall. The clicking/tapping occurs every time the ignition key is turned on for about 10 clicks turn the key off & it repeats the 10 clicks again. Its consistant & very annoying, anyone got a valid solution? Im an average DIY mechanic.any help wld be appreciated.

It’s the blend door actuator. Very common problem with these trucks. The plastic teeth strip out, making that familiar rapping sound every time the key is turned off. If you use the search box below the “ask a question” button, you can probably find several discussions about this issue.

Here is a diagram of your door motors.