I have a high pitched noise... sounds like from down pipe or exhaust (MBRP catted-Ford Racing catback) at 3500 and then again at 5000 RPMs... almost sounds like a little horn/squeak but it resonates a little. I'm thinking maybe down pipe gasket? Anyone experience this? I've had a couple mechanics check it... the first one couldn't replicate the noise (only seems to do it when car is warmed up) ... the second couldn't diagnose it. Any ideas?