Hey everyone, I have a 1997 gmc suburban 5.7L N/A, has about 80,000 miles, most of which was used for towing a boat or getting construction parts or other heavy loads.
When the car is on the freeway generally 65+ if the car Idles or coasts to a lower speed then it makes this Loud *** knocking/clanking noise, if the brake is engaged then the noise cuts off rather abruptly. I also believe turning might stop the noise(not sure on that one)

I know a good amount about cars, could this be the Flywheel or crankshaft bearing due to high rpms with towing?

any help would be awesome.

 

I don't know if this is your issue but I know on my 97 tahoe with a 5.7 I had to run at lest mid-grade or premium for spark knock... you might try a few tanks of high octane fuel to see if it changes the condition

 

a. 4wd?
b. what turns may stop the noise, left or right?
c. noise only on coast or deceleration not on accel? Nor if the brakes are applied?
d. does it sound like its coming from the front or rear or middle of the car?

 

It is a 4wd not Awd.
I think its both left or right, but not sure.
but yes the noise is only on coast or idling, Accelerating or braking puts it to a stop

it sounds like its coming from the engine compartment but it is very noticeable throughout the whole car.

I will also try the premium gas, thanks

 

IF its not happening under load its likely not ping or knock so high octane won't help... sounds like you might have a connecting rod coming loose...

 

I d check the locking hubs in the front. If its that electronic shift on the fly junk one wheel may be locked and the other free-wheeling. And look behind the steering knuckle also. The axle shafts arent the same length in the front so the shorter axle may may have a bad CV joint and or no grease in the boot. Jack it up and take a look.

 

Aight man, ill do that. thanks for helping

 

Kinda feared as much, thanks for the advice

 

If you can drop the oil pan a quick inspection of the connecting rods and caps to the crank is easy enough...
You may want to try a stethoscope or the trusty closet rod dowel to different places on the the engine block to see if you can identify any sounds while the engine is idling.. if something is loose inside the engine it will also make noise at idle it just may be harder to hear....
Its actually pretty cool to "hear" the inside of your engine working...
and I will state the obvious of "Be careful when placing anything on the running engine that is attached to your body"...


FYI harbor freight has them for $8 or less http://www.harborfreight.com/mechani...ope-41966.html

 

"Be careful when placing anything on the running engine that is attached to your body"... Lol, you just made my day.
but ya your right ill try hearing it when idling
thanks