Metallic rubbing sound at low speed

This just started yesterday. Sounds like the fan hitting a thin piece of metal briefly from 0 to 5mph then disappears. Won't do it once up to speed or accelerating.

Then I started it this morning, put in reverse and it was quite loud backing up. Almost like a shroud vibrating type sound. I'm stumped.

07 Tacoma 4x4 Doublecab 4.0
49,200

Thanks

edit:
I did notice the shroud above the exhaust pipe was a little loose but can't possible see how that could be it.

 

i had an acura a long time ago that the heat shield above the cat got loose and would make a sound like youre describing... it was rpm dependent because of the vibration and harmonics. if that thing is loose, tear it off or tighten it up!

 

Its the baffle in the Cat converter. This sucks.

 

Good news its covered under EPA warranty 8 years 80K miles. Dealer has to fix it on their dime.

 

Its the Air Filter housing that is rattling. Had the taco inspected and thats what my mechanic told me. Have a K N air intake system on the way so I haven't tried to dampen it.

 

Looks like you found it, but just to add to the list of possible metallic rattling sounds, here's mine: 1999 Tacoma, 240k miles, upstream 02 sensor was loose and rattled because the nuts rusted off. That took forever to find because it only did it under very specific circumstances at first.