Unusual rattling noise

[Waterburly]

I'm getting some unusual rattling noises coming from my 4Runner when I'm coasting at 40+mph or when I'm driving with slight throttle at like 15mph or so. It does not rattle any other time. I cannot determine exactly where the noise is coming from though. It just sounds like a lot of vibration causing the noise, but I don't understand why I can only hear it under certain conditions. Any ideas at all? Thanks.

[Home Built]

Check the u-joints.

[XJ_ranger]

harmonics

in my buddies 85 4runner, the back seat used to vibrate at a certain MPH - and when the speedo went out, we actualy relied on that to know we werent speeding...

[Waterburly]

I'll check the U-joints and see if that's the problem. If it is harmonics then I guess I'll just learn to live with it. Thanks guys!