Rattling on Acceleration

[numero9]

90 Geo Prizm Base, automatic 3-speed trans, A4-FE engine. 150,000 miles. It's getting around 30mpg.

There's a harsh sounding rattle under my hood when accelerating from a stop. I only notice it for about 5 minutes from starting the car up, then it goes away. However, it happens every time. The rattle isn't shaking the car violently but it's clearly loud and under the hood. It gets worse the harder I press down on the accelerator, too. I've been noticing the temperature gauge on the dash getting closer to the red than usual lately.

I just drove the car on highway speeds the other day for about 100 miles (a longer trip than it's used to).

Recent repairs: I just had my midpipe replaced and it doesn't sound like an exhaust issue. I just changed the oil and filter the other day, too. No changes in oil weight, brand, etc. Spark plugs are relatively new, gapped correctly. Transmission fluid is cherry, but I don't know about the differential fluid.

Any ideas? I want to get it before it gets worse.

[RED91SR5]

Originally Posted by numero9

90 Geo Prizm Base, automatic 3-speed trans, A4-FE engine. 150,000 miles. It's getting around 30mpg.

There's a harsh sounding rattle under my hood when accelerating from a stop. I only notice it for about 5 minutes from starting the car up, then it goes away. However, it happens every time. The rattle isn't shaking the car violently but it's clearly loud and under the hood. It gets worse the harder I press down on the accelerator, too. I've been noticing the temperature gauge on the dash getting closer to the red than usual lately.

I just drove the car on highway speeds the other day for about 100 miles (a longer trip than it's used to).

Recent repairs: I just had my midpipe replaced and it doesn't sound like an exhaust issue. I just changed the oil and filter the other day, too. No changes in oil weight, brand, etc. Spark plugs are relatively new, gapped correctly. Transmission fluid is cherry, but I don't know about the differential fluid.

Any ideas? I want to get it before it gets worse.

I know you say it doesn't sound like an exhaust leak but everytime this happens on my sedan it ends up been a loose down pipe to exhaust manifold connection. See if the exhaust donut is blown out at that joint and if it's loose. This is what I'm thinking it is, considering it goes away after the car warms up.

[90 GP]

It's probably carbon buildup inside engine causing engine pinging. Check the timing to make sure it's adjusted properly. Try a tank of 89 octane gas. If not been done replace thermostat and flush cooling system and check electric cooling fans for your running hot issue.

[numero9]

I filled up with a higher octane, no difference. One thing that I notice now is the car does it after each start up. I went to a few stores after running it for a while and without fail it did it each time despite the engine being hot still.

The temperature issue seems to be okay, it's stayed in its normal range all day. I tried to look for any loose connections and didn't notice anything. I'll have time later tonight to get a better look, though.

It does it when it's in park and only on accelerating, too. When the pedal isn't down, it doesn't make the noise. Still goes away after around 5 minutes of driving at any speed. Nothing else noticeably wrong though.

Anything else that it could be?

[numero9]

Alright, I got a better look and didn't see/feel anything loose. Parts of the exhaust are certainly rusted but I didn't see any major holes either. It does sound like an exhaust-type of problem by sound alone. The weave material that covers part of the pipe fell off sometime between the last time I checked. Is there any harm in letting this issue go for a while if it is indeed a rusted out/loose pipe?

[90 GP]

If the weave on that flex pipe is rotten the pipe itself is probably on the way out. The rattle could also be a loose/broken clamp in that region or the heat shield on the cat converter. Might also want to check to see if the shield around the exhaust manifold is loose.