I am a newbie, 2004 Mini Cooper S. 6-speed manual. Upon diving i can feel a slight jerk feeling when I accelerate then easier off pedal then acccelerate again. It feel like the motor mounts are loose. THe steering wheel is tight. THe front wheels have no play. Not sure what to check. Also upon reaching 50mph I can hear a sound that doesn't track engine speed. THe sound do not change if the tires roll on a different road surface. Sound like something resonating on the frame near the rear of car. It starts around 50mph. Any ideas?

 

How is your exhaust holding up? Are the mufflers rusting out? Have you checked the upper muffler clamps to see if they are still intact? They have a tendency to rust out and break, more so on the driver side. You may be getting some harmonic stuff happening back there because of something loose or rusted out.

 

mine seems to be doing similar... i have 71k on mine.. im sure original motor mounts. curious on this too... subscribing to the thread...

for the sound at 50 mph... mine has a thumping sound at this speed... although im pretty sure this is just from a flat spot in the rear wheels when i locked up the tires when my ABS was disabled

edit: reread your post... i'd check your exhaust as recommended above... theres 4 supports in the rear and 2 in the center.. these rubber mounts can fail.. and the metal support hangars rust out too.. at least on my '06 r53

 

Thanks.....THe forum rocks

 

THe problem was a bad tire. Changed all 4 tires, problem fixed

 

I would guess worn out front Lower Control Arm (LCA) bushings.
You can replace with stock rubber or aftermarket polyurethane.
http://www.waymotorworks.com/control...0-r53-r52.html
http://www.waymotorworks.com/powerfl...shing-kit.html

If LCA bushing needs to be replaced you might want to also think about replacing the ball joints. Here are both in a kit:
http://www.waymotorworks.com/powerfl...ll-joints.html

If you have a mechanic do the work you should be able to provide parts if you decide to use aftermarket parts.