Exhaust gets louder after warm up??

The engine is just loosening up as everything reaches operating temperature and clearances.

 

Any chance you have a leak between a manifold and a head? In many instances the exhaust getting louder as the engine warms is indicative of an exhaust leak-- things expand due to the heat and a small gap becomes big enough to notice.

 

In my car, if it is really cold, then it starts out at one sound level, then increases as the EGR opens. When the EGR closes 5 minutes later, the level decreases. Otherwise, I don't experience the first stage.

 

Well, the fuel/air ratio is changing. Cold engine gets considerably richer mixture which then leans out to normal air/fuel ratio as the engine warms-up. The sound will certainly be different between the engine running on rich and normal mixture. This difference should not be much but it could be, in your case, amplified by the type of exhaust system that you have.

My 348 has a noticeable, somewhat rough (a bit like pulsing-rumbling), sound coming from the intake openings for the first few minutes from cold then it suddenly stops. I found out that this is due to extra air injection system being active during the initial warm-up phase.

 

its normal for the exhaust tone to get a little stronger after warm up.
but I would just use my hand (feel for airflow) to check all the exhaust joints for exhaust leaks just to be sure all is ok.

 

Mine does the exact same thing. When I have my test pipes on it's even more noticeable. No leaks I've concluded it's just a lower resonance due to Lower rpm. When it's in the garage warming up items on my shelves go from still to vibrating once it's all warmed up.

 

I think we have the winner right here. When the engine gets to a certain operating temperature the ECUs will switch from closed loop to open loop. I'm sure there is enough difference in the mapping that you can easily hear it in the tone of the engine.

 

I would think when cold the engine uses more fuel and has more resistance, the air valve will also allow more air in the engine to reach the same idle speed, so in this way you might expect less noise when some time into warming up.

It might simply be "growth" of the exhaust system, changing the resonance frequencies of it. The exhaust on my 456 grows about an inch, visible by the tips protruding clearly further from the bumper when it's hot.

 

my 355 does the same thing. The noise level increases and the sound is lower in frequency. I think the muffler attenuation changes as it gets hot. btw, I run a tubi muffler with hi flow cats and stock headers.