Strange noise from hot engine after rebuild

[Einar Irgens]

Bought a rebuilt engine for my 911 five years ago. After severe problems with it due to an incompetent builder, the cylinder heads had to be dismantled and refurbished again last winter, this time by professionals. The compression is now as it should be, and the valves no longer leak. The engine has been installed in the car again and has been tested for running, and unfortunately an unfamiliar sound appears as the temperature rises. This is of course very conserning.

Here is a video that hopefully gives some indication of what could or could not be causing it.

https://photos.app.goo.gl/YAp1So1kC9SPMTn36

Keep in mind that the teardown is only partial. The cylinders and pistons were left as they were.

Any competent advise will be most welcome. Wild guessing I can always do myself.

[Henry Schmidt]

It is very hard to give an accurate opinion from a video but just off the top of my head, I would suggest a piston to valve clearance issue.
A complete review of the work done would aid in this assessment but first thoughts are often pretty close.

[Einar Irgens]

Thanks, but the engine was driven about 2000 miles without making that noise prior to the latest teardown. No changes have been made to heads, except grinding the valves and seats.

I think it sounds like to surfaces rubbing against each other on the outer side of the engine, but the heat factor makes me worry it could be something more serious.

[Henry Schmidt]

Valve to piston clearance can change by very minor changes the cam timing.
If you're certain it is a rubbing noise, simply remove the fan belt to eliminate the alternation and fan rotation from the equation.

[kens_74_911s]

Really hard to tell from that audio. Can you get a better mic set up? Also if you can get lower and move from bank 1 to bank 2 on the sides upper and lower vs picking up exhaust noise it may help.

[Tom_in_NH]

Fan scraping, alternator bearings maybe.

[Einar Irgens]

The car is sitting in a garage far away, and the guy working with it has been busy with other things lately. The plan is to detach the fan belt, but I reckon that any object directly or indirectly connected to it ought to be unaffected by the working temperature.

The alternator was new four years ago, and has been driven only a couple of thousand miles.
The fan has not been touched during the latest work.

It may have something to do with the hydraulic chain tensioners, since their function to some degree is affected by the oil temperature, but a loose chain doesn't make that kind of noise, does it?

[Einar Irgens]

The mysterious sound vanished mysteriously, so it will remain a mystery.
Thanks for all advise.