About a month ago I noticed the muffler on my stock exhaust system was starting to be louder and louder. First it started out with a light noise but now it sounds like a loud loud mower at 20 mphs and up. Anything under 20mphs and it's quiet.

Took it out to a muffler shop and they inspected the exhaust system and found no leaks or holes. They said it could be a bad exhaust resonator but they said it's extremely rare for that part of the exhaust system to go bad. They could weld in a new resonator but made no guarantee that was the problem because according to the muffler shop, they rarely see a bad resonator on a stock exhaust system.

 

Not sure how they can feel confident there are no leaks since the noise happened at 20+ mph. Perhaps a cracked exhaust manifold that needs some pressure/speed to show??

Testing static when the problem only occurs in motion is a recipe for frustration. Perhaps recruit a driver and stand by the road, have the driver approach you at 10 mph and accelerate as they get to you...retry at 15, then 20, etc...see what you can determine.