I have a flowmaster 40 on my truck and when I start it up it sounds great, nice rumble and when I accelerate up to second gear it gets louder and sounds awesome, but after around second gear it seems to lose all tone and go quiet. I havent heard the cat rattling but I've heard that sometimes when they go bad they can reduce air flow and you can tell by your exhaust not being as loud. Truck is also seeming a little sluggish. Any advice?

 

have it pressure tested at a muffler shop. my cat was really clogged and when i got a new one, the truck was 2x as loud.