knocking noise in 5th gear at 70 mph

[mitch244]

I just bought my 2006 mazda3 with 120000 miles on it, I noticed that when i get up to 70mph in 5th i hear a knocking noise that seems to be coming from the passenger side. As far as i know the engine mounts have never been replaced and that seems to be a problem with some mazdas. This is also my first standard car and i seem to have more difficulty shifting into 5th than any other gear,( i sometimes shift into 3rd by accident but try and quickly shift back to 5th) I am not sure if this is a problem with 5th gear or just my problem. Any help or diving suggestions would be greatly appreciated!