any ideas?

Do you have aftermarket down pipe?

Chances are its that rubbing/vibrating on the crossbrace

Could also be a lose heat shield if you have one, but the down pipe is a good bet

Exactly what i was thinking. I have a turbo back and just finished the catback yesterday and my downpipe at idle and lower rpms hits that cross brace. Just gonna thow some extra washers on it as ive heard this can fix the problem cuz it drops the brace a bit lower

downpipe hitting crossbrace. I put a couple washers between the xbrace and the xbrace mount to lower it about an eighth of an inch. No more rattle.

It could be rubbing the front sway bar which is above the downpipe. When I had a Megan downpipe it would hit the sway bar

My crossbrace gets bent all the time. It's the lowest point on the car. If you're lowered you will hit it. I don't know if I'd put washers on it. It's just going to hit more often and harder the lower it is. You'll still have to take it off and bend it back if you hit something washers or not.

I have to take mine off every once in a while and bend it back. It's pretty easy to bend. Just support the ends off the ground and tap the middle with anything heavy. We have bad roads in Alaska.

Wow those roads must be brutal. I live in Wisconsin and I thought our roads were bad. I only drive in the summer but still. Anyways I just put two washers under each bolt hole today so a total of 6 washers and I drove it alot to see if I would hear it on hard bumps where I normally would and when my engine was hot and had a low rpm when I would hear it before. And I am glad to say it has cured the problem 100%.
My crossmember it completly straight though and in the 2 years Ive owned my evo 8 i have NEVER hit it on anything. I never take my car off road tho. I have a DC 3" downpipe w/cat delete and a megan 3" catback. before it rattled hard and my girlfriend would ask why this why that. But now I hear nothing from my car or the mouth in the seat next to me

Are you stock ride height? I'm on swift springs and I can't even go over some speed bumps. Usually it's a rock on a gravel road that hits the crossbrace. The front tires go in a dip or rut in the road and it only takes a 2" rock to hit it. I've never hit it that hard usually I'm going really slow if the road is rough. I've gotten used to driving a lowered car.

Nope, im running on Tanabes and if my memory serves me right which I know it doesnt, I think its a 2" drop in front and 1.8" drop in rear or vice versa. Im able to roll slowly over speed bumps. I also have a very low CF front lip too. I have had some very low cars before much lower than my evo so I guess im used to watching the road very very carefully. But, now I probably will go out and hit the worlds largest raccoon tomorrow and

HAHA every road is different every car is too. Either way it worked for me. Just a suggestion. I am a little jealous of the Alaska views you get there man. that place looks fricken amazing.

Exhaust shield is probably bent and touching. Just get under there and straighten it out.

Is the braid worn out/stretched at the flex pipe?

had the same problem, needed to tear the top heatshield cover off my Buschur HFC.

Get under the car right up near where the driveshaft meets the tranny. There's a dust cover there that is notorious for coming loose. I ended up just cutting mine off after about the 3rd time it came loose. If it sounds like a bad throw out bearing, then this is the culprit.

Im gonna try this too I think it's the down pipe also