I hear a rattling noise under my 1999 Subaru Impreza, hatchback, AWD.?

The noise, well open the hood when car is started and in park. Have a person rev it and find by ear. It's to difficult to guess. I have had enough subarus to say there is not a known object to start to rattle.

The fumes maybe your o2 censor went and burning rich or lean. Also not 99s are known for your MAF (mass air flow sensor) sensor to go bad (located upstream of airfilter.

CV joint you can buy a new boot but the labor involved you myswell get a new cv joint. Ebay has em pretty cheap. Search under subaru half shaft and/or subaru axle. They never list model year or model name in titles so you may have to look in each one as most fit many other models. OR you can get a boot that you put on without taking everything off, most autozones,pepboys,etc have these. They have an opening that will screw shut. If you ignore this and CV starts ticking, go buy these greece at walmart it comes in a slim can that is black and has pictures of molecules on it. It comes with an attachment tube like WD-40. YOu can ad dthis greece to prolong it or if it gets really bad when you get wheel wobble it will stop it so you dont crash and or break it on the road.

My brother got a AWD Eagle Talon and the same thing was happening to his car. This rattling noise could be that the muffler pipe is vibrating and hitting against the car...also since it's a all wheel drive...the drive shaft from the front to the back maybe loose...and this could be that the gasket or ring from the front is missing or wearing away and this will cause the drive shaft to rattle when the car starts moving---this could be the problem only if the rattling noise come "on" when the car is in gear and starts to move at the instant. hope this help...

There is too much that could be going on to narrow it down on-line. Make sure you don't have a rock on the cat heat shield and if this rattle happens while the car is parked/still, then you can eliminate the CV joint as a source. If it continues, you need to have it put on a lift and inspected to find out the source.

anything older than 1 year old rattles... probably nothing...there is a "shell" around the converter, it comes loose from the actual inards of the cat converter... it really is nothing to be worried about...we had a small import truck that did that...it did it for the 5 years we owned the thing...besides being a little annoying it is absolutely nothing to worry about...good way to see if thats what it is...get it running when it is rattling... get a 2x4 and push on it...the extra tension you make on the outer cover will make it stop, then you know its just a loose outer shell on the Cat...

I had to do 2 CV's in as many years because I didnt take it seriously when I was told the boots being torn was going to kill them...

but that CV boot needs repaired ASAP or your CV is going to go

CV might be piling up, grab the shaft and wiggle it. Get that boot repaired or it will fail prematurely from dirt and water getting in there. There shouldn't be alot of play. What type of fumes? Burning oil, antifreeze or something else?

I would look at the exhaust very close again, alot of subaru's tend to break, one of the exhaust hangers midway back on the car... i have seen alot with pebbles stuck in the heat shields...

get that boot replaced. or your cv joint will break down sooner than you need it to. also, there are more than one heat shield surrounding the catylitec converter. make sure they are all tight.