I have a Trane 13i split system. It is making a "shuddering" or "jackhammering" noise when it shuts down. It only does this in heating mode, not in cooling mode during summer. Local contractor has tried putting a magnet on reversing valve, then replaced the reversing value on the outside unit, then put a larger bleeder value on the air handler...all to no avail.

Any idea what the cause could be?