JMHO, but it sounds to me like your mechanic is on the right track.
When the car is cold the entire exhaust system is contracted. This maximizes any looseness which may be present throughout.
As the car warms the exhaust system heats up quickly and expands, closing spaces and reducing looseness which might have existed when the system was cold. This would explain why the car makes more unwanted noises when it is cold.
My suggestion would be for you to raise her up in the air, get under there, and go over the entire exhaust system slowly and carefully. Try to shake everything and repair (tighten) anything, pipe, connection, or otherwise, that moves.
Loose exhaust system components can make all variety of noises, especially when you step on the gas.
Less likely would be a loose INTERIOR component within a CC or muffler. I would not expect this, but it's an outside possibility.
Good luck.
Got some anoyine problems with my catera. 
sounds that it makes. So I Was wondering if anybody could help me or ever had this problems. First of all I have this constent ticking that comes from the front right passenger side. It sounds like it was from the motor but somebody told me it was my manifolds. Then some mechanic told me that it could just be the bolts from the manifolds to the exhaust are loose. So I dont know what it is it sounds really loud when I first start the car for the day. And as it warms up you could here it less. And you could here it when I step on the gas As I drive. And the seconed thing is that my muffler rattles when I put the car in drive while im still and only when im still also more when my a/c is on sometimes just when its in park you can here it. Its very anoyine I know its alot but if anybody could help me please Thanks
