Hi, I had a whole new exhaust fitted yesterday with Kwik Fit (including cat). I started the car this morning and for the first 10-20 seconds the exhaust is making a rumbling noise on start up and for the first few revs. Best way I can describe the noise is like a boy racer exhaust. I work as a community carer so the car is stop start all morning and it was making this noise for the first few hours until eventually going away. However having been idle for a few house this afternoon I went to start it up and it made the noise again. Is this normal? Any help would be appreciated. I did take it back to kwik fit after work and they could find nothing wrong, although they didn't hear the noise I'm describing as by that time it had stopped. Thanks