USNA93

My 2005 TL 6MT has 200K miles on it and still runs great. Latest oil change and inspection at the dealer last week showed everything looked good (except rear main seal leak).
But driving it this week on the freeway I heard what I can only describe as a constant "thud thud thud"....or like when a semi truck does an engine brake (but not as loud). Here's the thing: I don't feel anything in the driving at all...not in the steering wheel, handling, etc....it's ONLY audible.
AND it only happens when I take my foot off the gas above 40MPH. It's a faint enough sound that I can't hear it with the radio on.
Two last data points...No codes after running my OBD tool. And after I had the tires rotated today the sound went away (it was making the sound on the way to the tire shop).
My local shop said it could have been an object in the tire/wheel that came out during the tire rotation but that doesn't explain why it was only occurring when I took my foot off the gas.
Just curious if anyone has experienced this and can provide any insight.

USNA93

Update. As I was driving the car for a tire rotation, I heard the same sound as reported above. Had another appointment with an auto shop immediately after to inspect for the sound. After the tire rotation, the sound disappeared as I was driving it to the auto shop.
Been driving it for 2 more days and all sounds back to normal. I don't hear anything coming from the back tires either. All I can think of is that perhaps the balance got off and the tire rotation fixed it all. But not sure why the sound was only happening when I took my foot off the gas.
Oh well.