you might want to check to see if those garbled transmissions might be in the AM mode...and you're listening in the expected FM mode...
i had a similar incident here on 148.55 ...a freq used in FM by USMC here, that i had plugged into all my scanners for local FM monitoring....but during a massive Army training exercise for Special Ops night drop by 15 C-130's out of Pope/Ft.Bragg flying a racetrack pattern over the Carteret/Onslow coast, the Army Jumpmasters aboard the C-130's were using 148.55 AM !! to communicate inside the aircraft ( i know cause they were just above treetop level over my house in a long slow line....)
Just a good thing that no USMC ground traffic in FM on that freq QRM'd them !!
but i think MARS traffic is probably the best bet...but best to be safe and check out covert ops on discrete freqs hiding in plain sight !
Barefootdipole
Carteret County