Just happened a few days ago, turn the heat on after
starting my truck, and this loud humming sound
just takes over. Heat is fine, but the noise is
annoying. Could this be my thermostat or something else
getting weak?

 

Can anyone help please? Sounds like a low bass sound

 

Does it make the sound when the A/C is on?

 

Does it make noise with or without the blower on? If so, does it change with blower speed? My car had a panel underneath the dash that encloses the blower motor that would make noise at it had slipped.

 

It makes noise with a/c on as well, humming noise is louder
when higher speed of heat or a/c turns

 

You need to look at the blower motor and fan. Probably bad bearing in the motor or unbalanced fan causing vibration.

 

I'll try that. Thanks for your help!