My 355 with Caprsito (and cats) has just started making a slightly rumbly noise under load. As an example, i try to keep revs under 4000 until the car has warmed up. However i've recently noticed some resonance and general grumbling if the car labours a little at low revs in 5th or 6th gear. The engine note and exhaust however both sound their usual excellent selves once i start gunning it. Any suggestions where the problem might lie? I know the manifolds are a weakness on the 355, but not sure what symptoms to look out for?