Rattling sound only when in gear (D or R) even at idle

Maintenance/Repairs
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I have a 2004 Mazda 6i (4-cyl automatic), 73k miles and very well kept. Recently it has started to make a rattling sound; not painfully loud but a clearly noticable and sounds somewhat metalic. This only happens when it is in gear (Drive or Reverse) and does not occur in Park or Neutral. It does not appear to get louder with higher RPMs (e.g., as vehicle is moving). Not 100% sure on exact source, but I think it is coming from near the engine - on the left side where a rotating belt is mounted (it does sound louder when hood is popped). Any ideas on what this is likely to be? Really appreciate any support.

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Have someone check for a loose heat shield along the exhaust system. Either they’ll try to reattach it, or tear it off.

Tester

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Oh and to clarify, the sound appears to be coming from the front of the car on the passenger side (e.g., where that rotating belt is attached to the engine). Just wanted to clarify since I said left side and meant left side when looking at the engine from the front… it is on the passenger side.

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A loose heat shield is one thing to check. Another is the serpentine belt tensioner, pulleys and/or condition of the belt. If the belt is stretched to its limit or if one of those pulleys is binding the tensioner will rattle against its stop. So have a shop inspect all of your shields and your belt & belt system.

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Thanks Tester and cigroller. I will try to have the heat shields and the belt/belt system checked. I’m just glad the transmission wasn’t the first thing that popped into your minds; really hoping this isn’t a costly repair or a sign of anything more serious.

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Thanks again folks. It turned out to be the belt tensioner (good call cigroller). Went ahead and had it replaced today and the noise is gone (along with some of my money, but whatcanya do). Thanks.

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Thanks to you for taking the time to post a follow-up. People rarely report back so people here usually never know what became of a problem.

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Hey! I have a 2000 Ford Focus and I was having the same problem and it turned out to be the heat shield thank god! Just got it checked out and they took it off for free

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I have a rattling sound only when the car is in gear and the AC is on. Compressor?