I have just bought a 2005 V6 manual Mustang and at bang on 71mph im getting a back end rummble, when i increase speed it is gone, anyone have any idears? the car was standing for a while before i bought it.
Thanks.

 

Could you be more specific about the sound? If the tires weren't rotated they could be cupped or worn unevenly.

 

i get a slight rumble but i think it has to do with the RPMs.

 

In what gear, 4th or 5th or automatic.
More info to diagnose unseen over internet.
There is a TSB out about it.
Check your magnetic fill plug for
metal chips.
Your rear end left the factory without enough rear end lube.
Fill her up to the fill plug and the growl disappears unless
there is major damage already...

 

Does it seem like a mechanical rumble? Tires, rear end, brakes, unbalanced wheel.
Does it seem like a body rumble? Loose spoiler, loose bumper cover

You are in Rhode Island, it is hard to see/hear it from here in WI

 

I had a vibration/slight rumble at 71-73 mph and an aluminum driveshaft cured the problem plus 05s have had problems with the driveshaft, no u-joint clips, check it out. Check the rearend fluid level? Could be tires? Could be axle bearings? Could be u-joints? Might also be nothing at all. Find a good mechanic you trust.

 

My '05 had a vibration with the stock driveshaft right at 72 mph, but could be felt from about 68-75 or so. They replaced the driveshaft and it fixed it; of course that was under warranty at the time.
I removed and still have the good replacement driveshaft and will sell it, but shipping would probably make it somewhat costly, as it is a little heavy.