Sheet metal rattling noise

[matt1967]

Hope someone will be able to help me with two issues with my 98 legacy GT 5sp. First issue is lately there is a noise coming from the engine compartment, sounds like a loose piece of sheet metal rattling around. Sound is most noticable when letting off the gas to upshift at around 3000 rpm. I creepered underneath and nothing is loose. I am suspecting an internal problem rather than something like a loose heat shield underneath. Does anyone have any ideas? Car has 98,000 mostly highway miles and always driven conservatively, never raced or abused.
Second issue is we live in a hilly area and I would like to save the brakes a little when possible. Does it make sense to engage the a/c compressor to increase engine braking? One thought says yes as the compressor will be a drag on the engine (similiar to a jake brake). Other thought says it wont help or may hurt as I believe that the computer increases fuel flow to the engine whenever compressor is engaged to ensure that a minimum rpm is maintained at idle. The engine is generally turning at 2000 to 3000 rpm when coasting down hill in gear. Any help is appreciated. Guess I could do some test runs by noting how fast the car is moving at the bottom of the hills with and without the a/c on.

[Will2]

The metal rattling sounds like the shroud around the exhaust manifold or maybe the manifold it's self. As far as the braking situation why don't you just downshift?

[matt1967]

I understand the concept of using lower gears to achieve engine braking on the downhills. I keep the car in second gear but that causes the engine to race at slightly over 3000 rpm. Third gear causes the vehicle to build up too much speed.

[sha_zapple]

chances are this rattling is the heat shields on the header. Mine rattle and theyre pretty tight. Im jus going to cut them off soon.

I doubt engaging the AC would do anything as far as braking goes. Just downshift. BTW, 3k rpm is hardly high.

[SUBARU_TECH]

heat shields are most likely the cause.they do get alittle exspensive to replace for what they do.on customers cars i always just drill a small hole thru the shield near the end and you a speed nut clip slide on the end of shield over hole you just drilled.insert the bolt thru hole and into speed nut.the bolt will push against the exhaust pipe securing shield.