Thumping sound at highway speeds/Engine roar

[jtbwatson]

I'm going to post this on blazerforum.com and s10forum.com but figured that I might as well post it here too...

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..

[Frank Lee]

Put a real muffler on it so you can hear what's really going on.

[dlb]

is it 2wd or 4wd? how long has this been going on for? has it stayed the same since it started or gradually gotten worse?

tires were the first thing i thought of but it sounds like you covered those. if there are CVs, i would inspect those next. look for obvious damage like tears in the boots. then i would check all the wheel bearings. jack up each corner and spin the wheel, listening for growling, clunking, etc. while the you have wheel up, push/pull on it to check for play.

good luck.

[jtbwatson]

Originally Posted by dlb

is it 2wd or 4wd? how long has this been going on for? has it stayed the same since it started or gradually gotten worse?

tires were the first thing i thought of but it sounds like you covered those. if there are CVs, i would inspect those next. look for obvious damage like tears in the boots. then i would check all the wheel bearings. jack up each corner and spin the wheel, listening for growling, clunking, etc. while the you have wheel up, push/pull on it to check for play.

good luck.

It's 2wd. Just recently started about 3 weeks ago and it has stayed pretty consistent actually. There are no CV joints, but I haven't thought about wheel bearings yet. That'll be my project for today.

[jtbwatson]

UPDATE!!!!

While not using the best eco-driving techniques today, I was able to narrow it down a bit.

It only makes the sound/roar while the wheels are straight or if i'm turning right. When i aggressively changed lanes earlier, it went away completely. SILENT! but then returned whilst going straight again.

[CapriRacer]

It's possible we are talking irregular tire wear - which is caused by mis-alignment.

Irregular tire wear comes out as either a vibration or noise - which are essentially the same thing, just different frequencies.

Rotate the tires and see if it changes. If it does, then it is likely your tires are developing irregular wear and an alignment is in order.

Please note: My experience says that the alignment specs published by vehicle manufacturers is too wide - by half. The settings have to be within the inner half of the tolerance to prevent tire wear issues.

And you should be aware that once rubber is worn off a tire, the tire can't grow new rubber in its place. Any new wear pattern is on top of what is already there. So tires with irregular wear tend to continue that wear even after the source of the problem (alignment) is fixed.

[jtbwatson]

Originally Posted by CapriRacer

It's possible we are talking irregular tire wear - which is caused by mis-alignment.

Irregular tire wear comes out as either a vibration or noise - which are essentially the same thing, just different frequencies.

Rotate the tires and see if it changes. If it does, then it is likely your tires are developing irregular wear and an alignment is in order.

Please note: My experience says that the alignment specs published by vehicle manufacturers is too wide - by half. The settings have to be within the inner half of the tolerance to prevent tire wear issues.

And you should be aware that once rubber is worn off a tire, the tire can't grow new rubber in its place. Any new wear pattern is on top of what is already there. So tires with irregular wear tend to continue that wear even after the source of the problem (alignment) is fixed.

It ended up being the wheel bearing in the passenger side front tire. I ordered new wheel bearings from 1aauto, but after I install those I will be sure to take it in to have it aligned and rotated.