The temperature blend dial on my mom's '05 Explorer is not working.

When I turn it to hot, I can hear a soft clunking under the dash. Sounds like an old grandfather clock's pendulum. Same sound as when you change from vents to defrost, like a vent shutting, but it keeps making the sound until I turn it back to cold.

Sometimes it will "clunk" a few times and actually switch temps but mostly I'm stuck on either hot or cold.

Have checked fuses for both the front and back temperature actuator and they're fine. The back control works fine. Just the front one won't work.

Have considered vacuum hoses and also heard of a cold/hot pendulum (which is what the noise sounds like) but don't know where to look for any of this stuff.

Given it's intermittent condition, could a vacuum hose be partially off? Or does this pendulum thing exist and is it causing the problem?

Any suggestions or locations to check would be great as last week it was in the 50's and 60's and this week highs are in the 30's and 40's and it's stuck on cold...brrr!

Thanks