2005 Nissan Xterra weird vibration when accelerating

Hi,

I have a 2005 Nissan Xterra which seems to run just fine, however, I am noticing this strange kind of vibrating/fan-like noise that I hear when I am accelerating. It is pretty faint, and reminds me of a noise you would hear when driving over tightly ribbed pavement(but really faint). It seems to occur the majority of the time when I hit around 10mph and then somewhere between 20 and 30 mph, but sometimes just randomly @ any mph. I haven't noticed it above 40 mph. Sound lasts only for a second or 2. Sound/vibration seems like it is coming from directly under the DS front end. Any ideas on what this could be? It is an automatic 2WD Xterra.
 

?? Are any exhaust parts loose or close enough to touch at certain loads?

I think the problem has been fixed! I got under the car and noticed that one of the metal clamp/bands that holds the heat shield in place had broken. I got a clamp @ autozone for .88 cents and put this in the broken one's place. I haven't noticed the sound since. That's my kind of mechanic bill. Thanks!!!