Didn't know how to even begin a search for this problem, so I thought I'd ask...
When driving down the road in our 2000 RX300, there is a noticeable "thump, thump, thump" noise. I "think" it's in the front, but now 100% sure. However, if I either give it gas, or touch the brakes, the thumping goes away. So, it's only doing it while coasting. Anyone have any idea what the problem may be? Thanks in advance...

 

I heard a similar noise coming from the rear of my 02 RX300. Mine sounded almost likes a golf ball in a box. I needed to replace my strut mounts and with 150k miles I replaced the struts as well. Occasionally I hear a metal to metal contact sound from the front - I am certain I need to replace the fronts as well. I'd say you need to replace the strut mounts and while the works is to done, replace the struts as well.