My 2002 impreza has approximately 75,000 miles.  Today it started to make a noise when accelerating.  I don't know how to describe this noise--it's not a knocking,ticking or rumbling noise, and I'm not sure what a whirring noise is--to me it sounded like a suction of a vacuum cleaner.  When I pressed on the gas pedal the noise got louder--but here's the thing  it would stop and come back, so it wasn't continually making the noise.  In addition I thought I heard a very quiet noise when not accelerating, but not sure--couldn't pinpoint that in traffic. I'm bringing it in for repair on Friday but was wondering if anyone could suggest what might be the problem. I would like to be somewhat knowledgeable about this situation when I speak to the mechanic.

Is it a high pitch or low pitch noise?

 

Any recent work done?

Got a parking garage or an alley nearby? Drive with the windows down it may magnify it

The noise is low pitch. The most recent work was about 2months ago. It was the regular maintenance work. No parking garage or alley nearby--was thinking about that.

a growel?

Exhaust heat-shields rattling?

I listened very carefully this am as there was no traffic in the street. The noise I hear from the outside by the engine sounds like a hissing noise. Sounds different on the inside. Of course it stopped when I got to the Subaru service. They're letting it sit for a while and will do another road test

heat shields.  common, particularly in rust prone areas, happens all the time. 

the dealer might be a bad place to have that repaired - it could be extravagant in cost for something many people repair for $2 and 8 minutes.

 

a mechanical/movement related noise under load is almost always a CV axle (recent new axle installed would be suspect) or front differential.

check the differential fluid level and quality and look for metal shavings.

but i doubt you'll need to get past heatshields.