This car is a 2001 BMW 325i E46 with 150k miles. Well maintained and seen by a specialist mechanic in the last month.

Loud hollow rattle/clacking sound just started. Persists stopping and starting the engine. Present at idle and at speed.

Video of the sound: https://www.youtube.com/watch?v=3U1LmENYGa8

  • Welcome to Motor Vehicle Maintenance & Repair! It's really hard to tell on the video what you're talking about. I cannot discern what exactly you're talking about from your description. Jan 6, 2018 at 23:37
  • It's indeed difficult to diagnose just by listening to a video, but I'd go around and try to pinpoint where the sound seems to be coming from. If you can isolate the region, that will help quite a bit. Do you know when the most recent oil change was? Does it have an AC (and was that on during the video?) Does the sound change with the throttle? Do you notice any other problems?
    – Hari
    Jan 8, 2018 at 3:00
  • Does it sound similar to excessive valve play or valve ticking? Wach videos from knowledgeable people explaining those problems to compare them to your own sound. The sound in your video is not clear enough to guess.
    – Bart
    Feb 14, 2018 at 19:21

2 Answers 2


This turned out to be a failing belt tensioner pulley.


After listening to your recorded engine noise, I suspect your alternator bearings are the source. If you have access to a laser temp reader, check the temp of the alternator near the front and rear beariings. Compare that to an alternator on a similar sized car. Yours will significantly hotter. You can also put a wooden dowell on various points to isolate the clatter. Be careful as you put the free end againt you ear. Check pcv for clatter, top of valve cover, ac comp, ps pump, alternator etc. You have a TSB out on the HYDRAULIC BUCKET TAPPETS. (HVA) You can review the TSB at: http://www.bbbind.com/free_tsb.html


***Now, if it is your engine, buy a can of "Seafoam" from your local autoparts store. Add it to your oil, it clears clogged oil passages. Carefull not to overfill your oil level. The clatter should clear up within 5 mi of driving.

Sounds like a bearing, BMW TSB says it may be a tappet issue, I am also including a general oiling problem as a possible cause.

Good Luck

  • Is there a reason you suspect the alternator bearings vs anything else? Also, you recommend SeaFoam, and while I quite like the stuff, you offer no particular reason why that would address the issue the OP presented.
    – Hari
    Jan 8, 2018 at 2:57
  • From what I could hear on the video, it sounded just like my alternator did when the bearings started to go. They don't squeal, they have a hollow low frequncy grind that sounds like a clatter. I would be interested in hearing the engine when the a/c is cycled, ps cycled, headlights on/off, etc. just because the audio is similar to something I recently heard. If the owner can isolate to the valve train area, Seafoam Jan 8, 2018 at 4:14
  • If the owner can isolate to the valve train area, Seafoam will help. Seafoam opens up oil passages, thereby allowing lifters to pump up and run closer to tolerance. The BMW TSB I mentioned describes several scenarios where Seafoam would be an indicated solution - oil too viscous, binding HVA element, sticky valve train components. Jan 8, 2018 at 4:51
  • The TSB states that if the tapping is heard on various cylinders, the problem oil too viscous, low oil, etc. But if it stays on one cylinder, the HVA element itself has an issue (the TSB outlines a pipoint procedure. Interesting problem. I hope it doesn't take too long to bring it under control. If it is a stuck HVA element, valve(s) won't seat and will burn. Jan 8, 2018 at 4:54

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