1997 integra ls b18b1 bonestock with short ram intake

So this is what happened I was driving at 30mph and out of nowhere this
rattling noise came from under the hood pulled over and check under hood and saw that the T belt was so loose that it almost came off had the car towed and took it apart found nothing weird, tensioner still looks good, t belt look good, spring was good, what could have caused this?

 

who touched that last?

 

are you missing the plate that holds the timing belt in place on the lower gear?

 

Check that the tensioner is still attached to the peg (upper right) that it pivots on, and that the spring is correctly fastened to it's peg and the tensioner.. it may have popped off.

Confirm (as the previous poster mentions) that the two plates are on either side of the crank pulley so that the belt can't wander.

If all that looks ok - I'd be suspicious of the belt, that the internal strength fibers had started to break and the belt was stretching. Consider replacing it with a quality part - either OEM or something like the Gates 'Blue Racing Timing Belt' - my GSR uses the T184RB.

Alternatively - any water leaks? If your water pump was failing it may move enough to slacken the belt...

Mark