OK, I'll try to make this short.. I have a 1997 5.9 gas 3/4ton 4x4 with around 49k miles. The truck runs/shifts/drives fine, EXCEPT, when I am on very light accelleration (3-7% throttle maybe), maintaining speed on the highway (around 65) I hear a knocking sound that is hard to describe online, the best description I have is vibrate your tongue against the roof of your mouth, and that's what it sounds like!! It also does it AFTER strong accelleration, I.E. if you get into it leaving a light, it revs up to around 3.5-4k, and then you let off of it to allow it to shift, during the time while the rpms are up, and your foot is nearly off the gas, it makes the noise. I took it to 3 transmission shops, and they all say it is not tranny/torque converter related. If the truck is in gear, you can preload the converter at a stop light, (foot on brake, step on gas), no noise, put the truck in park or neutral, and sometimes around 2k you can get the noise to appear by playing with the throttle. I only have one other guess, on the way home today, I noticed the sticker than jiffy lube put on my windshield, it says 5w20 Oil!! :-O I'm gonna go out on a limb, and guess that is not the proper oil for this truck? Could it be making a lifter stick or something along those lines?? I am at a total loss on this one guys, I could really use some advise from ya!! Thanks in advance!!

-Dave

 

have you had someone play with the throttle while youve had the hood up to see if you can hear it and possably pin point it's location.

5w20 wont hurt a thing, you'll just notice your oil pressure might be a touch lower once the engine is warm.

 

yes, it sounds quite loud and it sounds like its coming from the front of the engine, possibly a timing chain slapping around or an accessory bearing?

 

ever used a mechanics stethascope???

if can tell you real quick what it might be.

but I think your going in the right direction, if the timeing chain tensioner has gone bad that can cause it, or heaven forbid something be wrong with your water pump

 

here comes my ignorance... Wouldn't I have to remove the water pump to get the timing cover off?? Why do you say heaven forbid something be wrong with the water pump?!?

 

yes, you would need to.

the reason I said that is if you have a broken impeller or something just bouncing around inside the water pump that cant be too good.

 

Check the clutch fan on the engine fan assembly. You can either check it by removing the serpentine belt completely and revving up to the point where the noise would normally occur or simply remove the fan assembly and bolt the serpentine pulley back on by itself and run it. Either way shouldn't hurt the engine if you don't let it run long enough to overheat.

 

This maybe obvious but is it a fuel ping or knock? knocking or pinging is caused by dirty injectors or low octane fuels, the sound is very low and subtle and sounds like what you are describing. If this sounds like what you are experiencing, try the highest octane fuel you can find (sunoco sells 94) and add a bottle of Top quality fuel system cleaner. This should clear it up. If you are allready aware
of pinging\knocking I appologize, if not good luck and let me know if this helped.

 

It could be you just might to give it a good tune up.
If you replace the pcv valve sometimes if that's what your problem is that'll make a rattle that you could obviously here from the inside.
GOOD LUCK!

 

Well, I have done everything listed above, and visited the dealer, spent over $300 in diagnostic fees, and NOBODY KNOWS! I am beginning to wonder if it could be a wristpin or a lifter or something along those lines. It is definately a knock, and not a ping. The truck is a 1997 with the 360 and 50000 original miles! I can't believe something internal could be wrond with the engine, but that seems to be the only options left! I did run the engine without the belt (to eliminate all accessories),and the noise is still evident. Anyone have any ideas? The noise is only apparent at specific times.. 80% of the time, it sounds, drives, runs normal, however at very light throttle (i/e/ maintaining 65mph on flat ground on the highway, the noise is most evident), it also knocks whenever the engine is "freewheeling", i.e. if you accelerate hard, (rpms up around 4k, and then back off the pedal allowing it to shift, it will knock while you are off the pedal, and it will go away if you accellerate again, or lift your foot completely. Could this be a wristpin, or lifter, or ?????????? I have been around engines for years, and never even heard of anything like this! Thanks for all your time help!

-Dave