i have a 1997 mustang, 3.8 v6, automatic transmission. it make a noise when i get the speed up to 40 to 45. it studders or vibrates. but not all the time. if i accelerate say from stop to say 60 to 65 it does not do it. thought it could be a fuel issue but not sure. any thoughts? just not sure what it can be. any suggestions ? thanks in advance .