weird sound from motor

On my 01 Tacoma i have recently heard a rattling sound coming from the left side of the motor. The sound will come about when accelerating from about 25mph-45mph then noise goes away. Also when this sound happens the motor seems to loose a little bit of power. I should also note that the truck needs a new cat. The truck has 244,000 miles on it. What could be causing this?????

 

When you accelerate is it a slight acceleration or really on the gas or doesn't it make any difference? Try doing it in 1st gear only, then 2nd gear only.

 

It really doesnt make a difference. But i will try 1st and 2nd. If it does it in 1st and 2nd also, is that bad?