I just noticed this yesterday... as soon as the trans shifts into 3rd, there it is. Let off the gas... gone Back on it... dang. It's extremely subtle. So subtle that I had to convince myself that it was real and not just exhaust rumblies. In fact, if the other two gears weren't so smooth, I'd probably never even notice it. This is the 904 in my 79 Lebaron. The fact that it comes on right at the upshift makes me think (fear) it's internal vs U-joints or a bad biscut, but I'll be looking at those too. Whaddaya think?