Two Strange Noises

'95 Tacoma, 3.4l, 5spd, 4wd, 176K miles, all stock as far as I know.
First noise is a metallic "rattle" (not a knock) that seems to come from the engine compartment. I can find nothing obviously loose in the engine compartment. This is only heard when underway. I have not been able to duplicate it when stopped. It comes and goes, does not seem to be either speed or RPM related, rather more related to "load" on the engine. While accelerating normally the rattle will exist until I shift in each gear, stopping during the shift, then when I steady up at say 40/45 mph rattle will go away, so long as I keep the throttle position steady. It will resume at most speeds up to say 70 or so whenever I accelerate. It occurs more often, but not exclusively at low ambient temps.
Second noise is a loud "grinding" sound (like a worn bushing/bearing in an electric motor) which only occurs at low (below freezing) ambient temps, but not always. This also only occurs when underway. This seems to be coming from the area near the far left side dash vent. This is neither speed nor RPM related and only varies (when it occurs) in loudness & pitch. It can occur when the heater fan is in any position, including off & changing fan position has no effect. Will generally do this one to three times over a 5-10 minute period & then not happen again for the rest of the day. Any ideas as to what one or both of these might be? TIA, Stan

 

re: first noise. Are you sure it's coming from the engine compartment? Sounds like your exhaust heat shield might be loose and rattling.

 

Wow... Mystery noises suck. They're just so hard to diagnose when they're so intermittent. I've got a couple myself, just learned to live with it, i'll fix it when it breaks. lol

 

If it is coming from eng compartment could be the shield on the manifold.

 

"If it is coming from eng compartment could be the shield on the manifold." Styx586
madjik- are you referring to the heat shield over the exhaust manifold or somewhere else? The shield on the manifold seems tight. Stan

 

I have had a noise, more a squeak/squeal, buts it's always in cold temps and when I let off the gas pedal. If I stay on the gas, don't hear it. I replaced my belts and that wasn't it. Thought it might be a driveline issue since I couldn't create it while stopped. I lubed my driveline last week and haven't heard it since. My u joints aren't loose, but it seems to have fixed the problem. Worth a try.

 

BenCo - Thanks for the response, but I did a complete fluid replacement & greased all the u-joints about 2K miles ago. Didn't make any difference in the noises, but my fuel mileage went up from ~ 15 to ~ 20 hwy. Stan

 

First one might be piston slap.

See if you can capture a recording of those. Sometimes that helps a bunch.