[T3] Knock Knock

Daniel Nohejl d.nohejl at gmail.com
Sat Sep 16 15:51:07 PDT 2017

Over the last few weeks, we’ve been trying to diagnose a noise…...

In Madison, I thought I noticed what sounded like a loose valve. A day or so later, we discovered that the connection between our heater box and muffler was loose. We tightened it and applied some muffler repair and the noise vanished. Now, we have a new louder noise that has emerged over the last few weeks. It’s been hard to diagnose as the noise is generally loudest at cold start and for the 10 or 15 seconds the engine is running before the high idle kicks in. To properly diagnose, I had to pinch off the AAR hose at the air cleaner so I could get a low idle for a longer period. I also required the assistance of Sean, whose ear has proven indispensable more than once. Anyway, here are the other general characteristics of the noise:

1. The noise is loudest at idle
2. Above idle, it sounds like a loose valve.
3. While cruising, it can barely be heard, even when driving next to a concrete divider on the highway.
4. After letting off the gas, the noise doesn’t return until just above idle. It definitely doesn’t start up right after getting off the throttle.
5. It’s clearly coming from the 3/4 side of the motor.
6. All valves are correctly set, there’s no excessive side play (solid rocker shafts), the springs are fine, etc. I’ve run the motor several times with the valve cover off and everything looks right. 
7. After unplugging the spark plug wires at #3 and #4, it’s clear that the issue is at #3. With the #3 wire unplugged, the noise diminishes by a good 80%

Here’s a video (credit Sean) that does a great job of capturing the noise


I know what all of this suggests, but I’m curious if what I’m describing rings any other bells for anyone. Or, if we know what it is, can it be fixed without splitting the case????



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