14 yr old gas furnace in attic had a little bit of a rattle noise with the fan running. I recommended it be repaired and inspected by a tech. He told the seller and agents that after he fixed the cabinet rattle he heard another noise and it needed a new heat exchanger. Seller agreed to have it replaced. Good thing I wrote it up. 
Anyone heard of a bad heat exchanger making some sort of minor noise?
Yup!
…and yes!
Positive or Negative Air pressure along the seam causes the whistling sound depending on design.
If the channels are filled with carbon it will be less noticeable.
The HVAC Tech will hear the problem right away, if he does not he will most likely have a hearing problem.
After installation of the new heat exchanger a full test should be done for leaks even though it is new.
I got lucky and was there doing a reinspect when they pulled the heat exchanger. The agents said it was ready but, you know how that goes.
The heat exchanger looked almost new!
The rattle is coming from the blower area. Probably typical bearing end play along with out of balance blades.
They said they were told to put the new exchanger in and not sure who the first hvac guy was…
Blades on the blower wheel often come loose.
So much so, the internal fan hub can spin inside the blower wheel.