Strange odor

A couple weeks ago I started smelling a strange odor in my truck.
Sometimes.
It smells like curry, or something similar.

I haven't noticed it much in the past few days, but today it came back strongly.

I was sitting here in near 90 degree heat so I'm idling.
The truck started getting cold and I turned up the heat in the bunk a bit.
That is when the smell came back.

I can smell it coming out of the bunk vents if the heat is on, yet not when it is turned all the way down.
And the effect is immediate, whether I turn up the heat or turn it down.
Turn down the heat and the smell is gone.
It doesn't happen in the cab, only the sleeper.

It's a 2013 Volvo.

Any ideas?

 

I've had this truck since it was brand new. I've been the only driver for almost 2 years.
The filters are new, changed less than a month ago, both bunk and outside.

Nothing changed from one day to the next. It just started to smell.
And now I know it is only when I have the bunk heat turned up. (NOT the bunk heater turned on, just the temp knob turned up past zero.)

 

Yes, well,

I'm hoping for a more technical answer.