I'm getting a clunking sound from under the hood when the brake is applied either going forward or backward. It started after the 20K service What is it?

 

Well, it sounds like a brake problem. Did they by any chance touch the brakes at the 20K service? If so, bring it back there,

 

Under the hood or at one of the front wheels?
Sounds like the pad is not seated correctly in the bracket. A simple inspection should find the problem. But that's at the wheel...

Good luck.

 

Sounds like the caliper pins need to be removed , cleaned and lubed.

 

in my truck, one of the front caliper pins came out completely. I was in the middle of moving cross country, I was lucky the caliper didn’t wedge tight against the wheel (it rubbed it enough to leave a mark).

Bottom line, this should be addressed immediately just in case there’s something serious happening.

 

Good point!

 

Might be lose wheel lug nuts (Low torque) or anti seize between wheel and hub surface, IMO