One problem related to engine noise has been reported for the 1999 Ford Mustang. The most recently reported issues are listed below.
I have a 1999 Mustang 3. 8l-v6. Recently, the car began to expel a white vapor from the tailpipe when the engine is hot, along with a vibration noise. This has happened as many as 2-3 times per day. I took the car to a Ford dealer for service and replaced an air pump. This did not correct the problem. I did some research on the internet and found that many people have had the same problem which turned out to be due to blown head gaskets. I discussed that issue with the dealer representative and they said that there is no record of that kind of problem with a 1999 Mustang. I have found that this is not true.