Kentucky Boy

I have a 2001 ES300 with 85000 mi. For a short time now, it has been making a noise while accelerating that sounds like a exhaust leak. The exhaust was inspected while it was on the rack and we could not find any leaks anywhere. I have inspected the Motor mounts and they seem to be OK. It has a similar sound that the old 4 Barrell carbs used to make when they were under wide open throttle. The car has always been garaged and it is in pristine condition. Can anyone suggest something that I can check?

I have a 2001 ES300 that I changed both Bank 1 & 2 Oxy sensors because of Check Engine light on. Light is now off but the milage has dropped to about 3-4 miles/gal less than before. As a matter of fact, the milage was excellent with the check engine light on. The sensors were Denso. The engine runs as well as it ever did, but just poor milage. Anybody have ideas on what to check?