2005 Limited manual transmission with 84,000mi. For about a year I've heard a slight thumping sound every 4-5 seconds at idle. It almost sounds like it's coming from the rear, but when I get out to listen under hood or rear of car, nothing. Today it was exceptionally noticeable, even felt it thump thru the steering wheel and shifter and it was about 2 seconds apart. Car runs fine, doesn't seem to be missing on timing. After driving 5 miles it's gone again. Ideas?