Acceleration Rattle

I have a 2001 Subaru outback that has roughly 200k miles on it but still runs great I got the car with about 160k on it and it has made this rattle since then. The noise sounds to me like some sort of a loose washer on a pipe or bolt that rattles when you accelerate. it only does this when accelerating hard and happens only if the automatic transmition drops to a lower gear. The rattle also only occurs between 4 and about 5.5k RPM and as soon as the trans shifts up it stops. I have replaced belts and have checked the belts rollers for bad bearings to find nothing. I have tried adjusting the heat shields on the exhaust and still have the rattle. Any help would be great.

could be something in the catalytic converter maybe? Or maybe a slight spark knock since it happens under heavy acceleration.

Does it make the noise with the car in park?

I don’t think it is either of these issues it really only happens under moderate to heavy loading it will happen in park but just barely and very faint. I have beat around on the exhaust with a mallet and have no noise from it so I ruled out any issues from it the spark knock issue I am not familiar with what this would sound like could you elaborate on how to check for this and when it would do it? on one forum I found that older model outbacks '94 or so had an issue with a separating fly wheel (not a fly wheel on an automatic but i forget what it is) and this separation was the noise? I’m just not sure exactly if this is it either and have no idea on how to test it. and if this is it is it dangerous to drive or not?

Are you sure it isn’t pinging you hear?
Have you checked the ECU for stored codes? Pinging if it is that should store a code, something like a P0382.

If it is pinging, it could be a malfunctioning Exhaust Gas Recirculation valve or even too hot a cylinder or too lean a mix. Or, if your owner’s manual requires preium and you’re using regular, too low an octane. Your knock sensor circuit could be malfunctioning also.

Check for codes and post back. Unless, of course, you are certain it’s a mechanical rattle and not pinging.

Could be a loose heat shield somewhere on the exhaust system.

Fill up with premium and see it the problem disappears.

definitely not a loose heat shield I have tried placing extra clamps on all of them with no luck.

I don’t think its pinging because it is more of a chatter type noise and doesn’t sound like the pinging I’m used to but ill take it to have the codes checked at auto zone and see if i have anything. Thanks.