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My 96' Mustang has started rattling when it idles, it started first when I just put the A/C on. Also. the Check Engine light is on. Now it does it all the time. I just had a tune-up a month ago. I had a oil change a week ago. They said all my fluids are good . Why is my Mustang rattling all off a sudden. Did something go wrong when they did the tun-up ?

Answers from the Automotive Experts

Amanda J. Pierce, MBA Chell, Thank You for writing into Askpatty!! To address your concern, typically you can visit your local auto parts store and they can tell you for free why you check engine light is in. If your engine "rattles" only when the A/C is on, it may be an A/C compressor clutch that is failing. You may be able to contact the facility that performed your tune-up and request a free inspection for a further determination. Hope this leads you in a more sure direction! A. J. Valle

Kerri Papajohn, Marketing Director at USA Sealants, Inc. Sounds can be deceiving as one man's rattle is another man's knock. If the noise is coming from the accessory drive belt area, the tensioner is probably toast. If the noise if from the top end, it might be timing chain(s). If the noise is from the crank pulley, it could be loose. If the noise is from the AC compressor, the idler bearing could be causing it. If the noise if from the rear of the motor it could be a loose torque converter or cracked flex plate. There needs to be further investigation as to the actual source. As for the check engine light, stop by any AutoZone or Advanced Auto and get the clerk to read the diagnostic fault code.

Kristin Brocoff, Director of Corporate Communications at If your check engine light came on right after you had the work done it could have been a result of what was done, but more likely it's just a coincidence. Regardless, you should alway have check engine issues diagnosed as soon as possible. I don't want to guess at your problem, but many issues can cause rough idling and rattling. For instance, a faulty EGR valve is one of the top 10 most common reasons for a check engine light to come on, according to CarMD's data. It costs about $280 to fix, and will cause rough idling, engine hesitation, misfire and poor fuel economy if you ignore it. I encourage you to get a second opinion with an ASE-certified repair shop, mechanic or dealership. Best of luck, Kristin Brocoff, Corp.

Lori Johnson, Owner and Instructor at Ladies Start Your Engines Dear Chell, A rattle can develop at any time. Something may have just come loose. I would have someone take a look at it with you so that they understand the noise you're hearing. As for the check engine light, you need to get that looked at as well. There are places like Advance Auto that will read the code for free. Then you can look up the code to see what it is. It could be something simple like a loose gas cap, or you may have a bad sensor somewhere. Either way, it's best not to find out what it is and get it fixed. Lori Johnson

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