l have a 98 Mercury Mystique,4 cyl and has 148k miles. Lately,my car has been doing 2 things. The first one is sounds like a diesel truck going down the road. l consulted a friend of mine and he suggested it could be the tailpipe has a hole or the muffler might be loose.l checked it out but everything is fine. 2nd one is l keep hearing a ''knocking'' sound in the engine. It is sporadic when l am driving but when l go up to 45 mph,It stops. It is driving me nuts since the last vehicle l had,the head gasket has blown which let me stranded. l consulted a mechanic and he mentioned to me that my car is over 10 years old and they start''seeping'' There was no sign of fluids leaking as l thought l had a oil leak. l recently had a transmission replaced,alternator,break pads,rotors and tires. I am trying to run my car to the ground but if worst comes to worst,l will have to find another vehicle.
Please help..

Never underestimate the importance of regular maintenance. I'd start by checking the spark plugs and timing belt. Your description of it sounding like a diesel truck makes me think it is an exhaust leak, which is not difficult to repair, but you've said it is ok, so l am assuming your diagnosis is correct. That is about as good as l can do over the internet with the info provided.

well from my experience when you fix one thing ten go wrong.the good thing is if you keep this car you will know the next time something goes wrong what is left.the bad thing is if you get another and it is not new something could go wrong and you would have to start all over again.