my wife's car is a 97 E420 w/ 89,000 miles on it - engine runs strong and pulls like a sonofa#@%&*, but when sitting at idle, there is a distinct "tic-tic-tic" sound. It's not constant, but occurs often enough that I recognized it. We are using 91 octane and oil level is perfect - local dealership suggested it may be a sticky lifter. Suggestions???