Thumping noise

Sat night, my 2002 Nissan Maxima (V6, automatic, 90K miles) started making an unusual noise.

The sound was something akin to what baseball cards in bike tire spokes sounds like, only deeper. The sound seems to be originating from what felt like the front middle, engine area of the car. It was only audible inside the car and only when I coasted or took my foot off the gas, even for a few seconds. The sound goes away when I accelerate. It was most audible between 35-45 mph and barely audible at 10-20 mph.

This morning it was louder and seems to be worsening. Any ideas what this could be?