I searched for this problem but didn't really find a good answer ( I could be searching for the wrong thing.) I notice it mosty at idle with the temp control adjusted to use some or all of the heat. My wife says it sounds like a little jackhammer.

When it rattles it causes my truck to slightly drop in RPMs. I suppose that whatever is causing the noise it putting a load on the engine at idle causing to drop RPMs slightly.

Can anyone comment on my issue.

 

Is the rattle sound coming from the same area where the blower motor is? Could be that the motor is wearing out and is starting to make noise. That may cause more resistance to get it to move, thus causing a drain/load on the system.

 

I haven't gotten down on the floor to listen but it makes sense. It doesn't rattle when the A/C is blowing just when I add the heat.

 

If it is the blower motor, it should rattle when the A/C is on also.
Maybe the blender door isn't closing all the way when you turn the heat on. That could rattle, I suppose, but I don't think it would cause a drain on the engine.

 

Is the blender door easy to get to or am I looking at tearing out the dash again

 

It is the door that seperates the outside air from the inside air. I do believe it is behind the dashboard near the blower motor, but I am not sure on your year truck.

I would have thought that some other members would have chimed in by now.

 

It could be the actuator that controls the blend door,it could possibly draw current if the door is binding or sticking.The actuator,(if memory serves)is just above the trans tunnel,(or hump)on the floor almost center just above carpet on the HVAC module,if it is there you can unplug it & see if the noise goes away.