Raspberry Noise coming from my car engine

I own a 2002 toyota corolla. My car keeps making a sound every time I accelerate, almost sounds like the car is doing a "raspberry". The car, i've noticed, also has been really bad on fuel consumption lately. I had my regular mechanic check it, and he thought it was the catalytic converter, so I took it to a muffler place he recommended; they said that my catalytic converter is fine, but that my valve cover gaskets were leaking and also my rear engine motor mount was broken. I've since had my regular mechanic repair these two items and thought that my troubles were over, but I noticed that the car is still making the same noise and the fuel consumption although improved just a bit, is still not what it should be (or used to be). I told my mechanic and he still thinks that it is the catalytic converter, so I took the car back to the muffler place and they again told me that my catalytic converter is fine, but that I still had a leak in the same place which might be due to a cracked manifold. Is this correct? If so, do you know what the average repair cost for this item would be?