It's a '96 with the 3.9l and an auto trans. It hasn't been driven regularly for 2+ years. I have driven it around the yard some but nothing above 10mph. After fixing a couple small items (blower resistor & splash guard), I was going to take it to get two new tires and a state inspection. About a mile or so from home, when the speed limit changed from 25 to 35, I noticed some engine noise. Mainly when giving it gas (not a lot) to get from about 20mph to 35mph. The gauges showed normal readings, pulled over and didn't see anything leaking or obvious problems under the hood. Started going again and it seemed to get worse as it was doing it at lower rpms. If I really stayed off the gas, it wouldn't make noise. I turned it around and limped home.

The oil level was at the lower mark on the dipstick. The oil pressure was good according to the dash gauge. In the past, I've let the truck idle for 20 mins or so at a time with no sign of noises. There was a slight burning smell when I pulled over and checked under the hood, but I attributed that to rodents/bugs/etc (pulled a mouse nest off of the air cleaner yesterday). It has some old gas in it, but I also put in a fresh gallon or two (with stabil) from the lawnmower/generator gas can as the needle was past the E. Adding 1-2 gallons brought it up to almost E.

Any thoughts what may be causing the noise?

Keith

 

It could be anything. So the noise doesn't come on at all under 35mph? Does it come on at idle or if you rev it up in neutral or park?

 

It seems to be load based rather than speed based, but not nearly as noticeable getting up to 20mph or so. I can rev the engine up to 3,000 rpm or so in park and it does not make the noise.

I've uploaded a short video to youtube. The noise is most pronounced near the end where the truck is cruising at about 20mph and I give it gas to go to 30mph.
http://www.youtube.com/watch?v=_9VsLI994gk

Keith

 

I used to have a grumble in my engine. It would sound fine while off the gas at low rpm's. Then when I got on the gas it would have that horrible grumble and transmission wouldn't kickdown unless I floored the pedal. I replaced the MAPS and Air Charge sensor. Now the truck doesn't act like that anymore.

 

This may have just been a case of old gas. I just dumped abt 4.5 gallons of shell 93 octane in the tank and took it out for a 1/2 mile or so drive. There was no noise. Yesterday, the noise was obvious on this same trip. I was planning on draining the existing gas first, but that idea was scrapped when I saw this tank doesn't have a drain plug.

Keith