Burning smell from air ducts only with heat on

What’s up fellas. I have a 18 TRD SPORT. I have done a ton of searching and cannot find anyone who has a concrete answer on this. I saw a post with someone that had the exact same issue and the thread went dead and no solution was posted.
I have a burning smell coming from the air ducts while only using the heater. If I use the AC there is no smell. If I put the temp at halfway there is no smell.
I have checked and verified no engine leaks and have changed the cabin filter. No dead animals or mice in the vents. No obstructions in the motor. Electrical is fine. No plastic bags melted on exhaust. I can drive around with temp in the middle and fan blowing with no smell. If I move to AC no smell. If I move temp to hot then it smells like something is burning.

I have searched high and low via forums and google and have also taken apart the truck and cannot find the source of the problem.

Any help would be greatly appreciated! Thank you all!

 

does it smell like electrical?

 

No, it’s odd like brakes when you ride them. It’s very odd and only with the heater.

 

How's your coolant level?

 

I have not checked. Will do in the am. Truck has 46k and has gone in for all normal service intervals. Low coolant can cause this? Hope that’s all it is. Thank you for the suggestion.

 

Are you still under warranty?

 

Unfortunately no

 

Welcome to TW!

 

Welcome to TacomaWorld

 

That ain’t good. Hope you find the issue

 

Welcome to TW!

 

Bump

 

Hows your coolant level