Posting this on s10forum.com also

I've got a 2000 Blazer LT with Michelin LTX M/S tires on it. The exhaust system has a Flowmaster muffler with a regular plain jane catalytic converter. Recently I have noticed a lot more engine roar as I am cruising as well as a slight thump while going down the highway. I can barely hear the thump while going <55 but once I hit 55+ it begins to become louder. Note, I can only HEAR the thump. Can't feel it at all. I took it to my shop (Service First if anyone is in the Spring/Houston area!!) and the tires were already balanced. I checked all of the tires treads to see if they were coming apart inside and they all look good. With the flowmaster on, it's hard to tell where exactly the sound is coming from. It actually sounds louder with the windows up than with the windows down. It's not my engine mounts (had my brother gas it up while parked - everything looks good) and it remains consistent while turning the wheel, I think. Whenever it's loudest, I'm not able to turn the wheel too much due to my speed.

So, what gives?? Shocks are good, front end was re-done about 30k miles ago, no check engine light is on, the vehicle is aligned, too. I'm really confused on what it could be.

Also on a side note- the vehicle has been getting less and less MPGs lately. I've got my amazon shopping cart filled with $400 of fluids, plugs, wires, filters, etc etc to help with this issue. It's time to re-do everything. Pretty sure it has never had its differential oil changed..

 

Check your fan clutch. That's always the first place I check for engine roar. I got a bad fan clutch out of the box for my '93 Jimmy when I replaced the water pump. I had a terrible engine roar and the mileage went down. After I got a second, more functional new fan clutch all returned to normal.

If the fan clutch is OK then check fuel pressure and leakdown. That's another common area of concern with excessive fuel consumption.