Raspy noise when engine coasting down? - 986 Forum - The Community for Porsche Boxster & Cayman Owners

Hi all, Trying to diagnose a noise on my 2000 S (~42k miles, manual). When the engine is coasting down I get a raspy/sucking/grinding noise. From inside the car it sounded like brake pads slightly dragging over the rotors but I also get it when the car is stationary. If I rev up to ~4k RPM and take my foot off the gas, there's a slight pause and then the noise starts until RPMs fall. It is not present at idle. Most noticeable when in 2nd gear and coming to a stop, but doesn't matter if the clutch is in our out or which gear. The slightest touch of the throttle makes the noise instantly go away. Half a millimeter in and it's gone. It's also not present on a cold start but appears relatively quickly as the engine warms. Because of that brief pause before the noise starts I assumed one of the clutched pulleys on the engine - I replaced the alternator pulley which *had* failed, and that solved some belt squeal I was getting when doing 3-point turns, but did not fix this noise. The AC compressor pulley feels OK and clutches on/off with the climate control button. All other pulleys feel good. IMS has been done, and due to the relatively low miles I'm hoping nothing mechanical. Otherwise the car runs beautifully. The fact that it immediately cuts off if I tap the throttle even the slightest bit and that re-snorkeling the car made it much quieter has me leaning towards some weird intake/throttle body/PCV system noise but I'm not sure what to check next. If relevant, the car has an aftermarket exhaust and resonated midpipes but stock headers/cats. Any thoughts? Here's a video: https://youtube.com/shorts/wvFSzaEDqjU?feature=share

Really difficult to hear, but I imagine I hear a chain? Doesn't seem like an obvious thing though, Could it be a vacuum issue instead?

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From under the car it does sound a little more like a chain, from in the car it definitely sounds more like an air/vacuum issue. I don't get the telltale startup rattle that variocam or chain guide wear usually has though. I need to first remove the belt and try to eliminate the AC compressor pulley/clutch. I did this first but with the firewall off I couldn't hear anything (belt on or off), so I need to remove the belt, reinstall the firewall, and try to recreate it with my phone under the car to compare the two clips. If I still have the noise I'll do a smoke test. Should tell me if it's a vacuum issue although I don't have any codes being thrown for a vacuum leak.

Quotes source post: 670605

If you had a vacuum leak, this would primarily be noticeable as a fluctuating idle speed. Because of the noise, I thought it might be a chain being pulled over a chain guide.

Pulled the belt when the car was warm and can confirm I still get the noise, so it's not pulley related. I did a smoke test and was getting some whistling around the center oil fill tube joint. I'd already addressed an oil leak here and replaced the clamp with a worm-drive style but I sealed this off with some Curil T2 to fix the minor vacuum leak. Still getting the noise but I finally found some other examples of the same thing. Three videos and two threads about it - all of them with a cheaper aftermarket exhaust with an x-pipe. https://www.youtube.com/watch?v=s9poqfZ4m2I https://www.youtube.com/watch?v=-N0gG2d5grw https://www.youtube.com/watch?v=gXju8ZJfySk http://986forum.com/forums/boxster-general-discussions/28505-strange-exhaust-noise.html https://www.986forum.com/forums/performance-technical-chat/21874-what-sound-exhaust-question.html Good think I kept the stock exhaust around