At about 3000 miles I noticed a vibration through the car. It was odd because everything was stock on the car at the time and I actually pulled over a few times to check my lug nuts to see if a wheel was loose. It only happens at 70 mph and goes away above and below that speed. Now that I have put 20's on the car it seems to have amplified the vibration. I have had all the rims balanced twice and even changed out each with a stock 18" wheel to see if there was an issue with a specific rim....no change. It's not a steering wheel vibration, that stays perfectly still but if I look over at the passenger seat headrest the seat is actually shaking. My thoughts are maybe an improperly balanced driveshaft or something of that nature because the vibration seems to be central in the car....like the driveline. Has anyone had a similar issue with their car? Especially in stock form.

 

Smooth and straight up to 140 MPH. j/k
No vibrations here - but you mention it's not through the steering wheel, so I'd bet one of your back wheels or tires is faulty.
Something a balancing might not catch.
Try the dealer and take out a tech. Then have them swap out the back wheels/tires and see if it goes away.

 

I swapped out each of the rear tires with one of my 18" stock wheels and it made no difference...only very slightly less vibration..but not much.

 

Please let me know what you found with this vibration... I just had my front two tires replaced and balanced to fix a steering wheel shake at 120km/hr.. When i drove home I was doing 140km/hr at one point and as soon as i get to about 135 there becomes this loud humm and vibration in the seat of my pants and i thought maybe it was the driveshaft or something too... It comes from the center/rear area and is kind of loud. sigh.

 

I should mention this is an 04 6mt coupe with 30k i bought almost a month ago.

 

well that should help a question about an 07' SEDAN.....

 

I searched all forum threads for "vibration"....He has the EXACT same problem that I have, its not like they are different manufacturers or comparing apples to oranges. Likely we are running into the same bloody flaw whether its a sedan or coupe... You aren't helping the thread either.

 

Well, had a 4 wheel alignment done and re-balanced everything...again. Vibration is still there. It's not in the steering wheel.....it really fells like driveline. I may put the stock 18's back on and take it in to the dealer and have them drive it and check the driveshaft to see if it has thrown a weight or something.

 

I'm thinking the same thing... Really interested in what you find though.