Hi all,

We have a 2001 Accord with 289,000 miles, V6. Regular oil changes, owned for five years.

In Oct 2022, we had the trans professionally rebuilt as it was exhibiting the usual slippage issues of this era. The new trans shifts great, no problems there.

However, this year the engine has developed a intermittent deep knock/thud at idle, which goes away either with RPM or under load. It is mostly noticeable from inside the cabin, but also perceptible when standing over the engine. Quick snap throttle tests exhibit no symptom of knock, but in fact the noise disappears when snapping the throttle. It only seems to happen at idle, it tends to fade in and out, and it remains the same volume whether warm or cold.

As far as I know, a rod knock would clatter on a snap throttle test, and piston slap should abate or go away completely with engine temp., but this does not.

I also suspected possibly an issue with the trans flex plate, but stethoscope-to-bellhousing I don't hear any pronounced noise. In fact, any stethoscope probing seems to indicate it's deeper in the engine, rather than up top on the valves or anywhere near the trans.

Any ideas or suggestions what it might be? Other diagnostic steps to take? Could bad spark or coils cause this sort of noise, without causing a perceptible miss?

Thanks in advance!

 

I've attached a short video of the sound - anyone who is willing to take a quick listen and help me identify this, or what you think it might be, is appreciated.

Thanks!