Intermittent Noise

I am hoping some of you may be able to help with this. I have an intermittent noise coming from what appears to be the back right undercarriage of my 2003 Suburban. It sounds like the sound your car makes when rolling over the warning track on the side of the interstate. What's bizarre is that it comes and goes as it pleases..it can happen as low as 7mph or 45 or 75. It can happen while turning or going straight I have never been able to purposely make it happen. The longest it has made the sound is probably 3-5 seconds at higher speeds and 1/2 second at lower speeds. I suspected the tires at first, so I brought it in and had them inspected and rotated by a trusted tire dealership and they said they could find no problems, and even though the tires were rotated, the sound stayed in the same place. It doesn't seem to be affecting anything but my nerves...any one ever heard of anything like this??? Thanks!

Is there any noticable vibration associated with the sound?

If there is, it is so very slight it is nearly unnoticeable or percieved, my wfe says she feels vibration in the floor.

Ok, Update
Don't know if this will help, but this sound happens without fail at 1000 rpm. I can actually make it happen if I hold rpm's at 1000 for a long enought period. Then once it begins I can keep it going as long as I hold speed at 1000. Any ideas?

This is exactly what my 95 turbo diesel has been up. At times it can sound like the engine brakes of a semi. Others it comes on loud like your riding the warning track. We did new tires and replaced a slightly bent rim, and it's still there. But not always, and you can never guess when it will come again. I think it happens more when the gas tank is empty, full I rarely hear it. I asked my husband to check if my freaking gas tank was going to fall off. BUT, hope this is a clue.....I did knowtice my muffler viberating while idling. Was an awful noise, and it would stop when I put my foot on it. So I have been hoping that's what it is. I'll keep an eye on your post though, wouldnt mind knowing exactly what it is.

This was happening at almost exactly 1000 rpm on mine, were you not able to make it happen at 1000rpm? BTW, our suburban has stopped this vibrating on its own, after I discovered I could make it do it if I tried, it stopped and hasn't done it for weeks now!!!!
Probably will start immediately after I post this, now that I have said it quit!!!