Alfa40

Listen to this recording of two odd sounds in my car.

12 and 14-second mark (audible for much longer and a lot more prominent in real life):
When cold, my car makes a light rattle when accelerating in second gear. I never thought much about it as it usually goes away with a warm engine. It appears to come from the passenger side and sounds like a heat shield vibrating (which would not change when hot, I suppose).

40 to 50-second mark (not quite as bad in real life, the microphone seems to pick it up nicely, though):
Recently, the car started making clunking noises when rolling in second gear (no gas is given). It also started to produce a light rattle when you turn the engine off (no rattle when you press the clutch). This is a much more worrying sound, but from my literature here, it seems the two might be connected.

From what I've read, this points towards either a worn dual mass flywheel, a clutch throw-out bearing, CV Joints or transmission mounts. My engine mounts were changed recently to OEM RS mounts. Any ideas, anyone?

pp000830

If you look for love hard enough, you will find it and she will be an expensive date.
I believe the first sound you describe is just when at certain speed and in certain gears the pulse from the engine pistons synchronizes with the sound that a gear in the transmission is making due to gear tooth engagement constructively add together to create apparent noise. It may be slightly louder as the transmission and/or its mounts age. Although highly unlikely, If you have a dual mass flywheel issue it can be definitively tested for by using the rotational sensor to see if the rotation timing jumps around a bit rather than providing a consistent timed pulse.
The second noise I am not sure about. A loose or worn engine or transmission mount? Something loose under or in the car or even something rattling around in the trunk or glovebox? When a clutch is engaged and the clutch components are spinning independently from the engine certain clutch designs can make a little noise. The dual mass flywheel or the clutch could be rattling a little however a little rattle doesn't mean they are not functioning properly or worn out.
All the above noises can be amplified as the car ages and sound insulation, body seals and rubber mounting bits show their age. It can also dramatically make sounds more apparent if one has altered related stuff in their car. In your case the transmission mounts.
Andy

mike cap

I didn't hear much at the 12 second range but did at 40 seconds. You said that the RS mounts are recent? Any chance that the new mounts are an issue? Loose perhaps? Did the noise start at the same time? I know this is obvious. The RS mounts (I have them) will allow a noticeable increase in running gear noise. You seem pretty tuned in to the car and of course if the noise changes with driveline RPM their are the usual suspects as Andy mentioned.

There isn't really any tin work under the car on the passenger side.

Alfa40

Ok, I took your advice to heart and tried to let things be for a while. We can get obsessed with these noises and there really might be nothing to worry about.

But then, the first noise got worse and worse. Yesterday, while driving in second gear with a somewhat warm engine up a hill (not spirited, the noise starts in low rpm already), the noise was so loud that my girlfriend, usually oblivious to any mechanical sound short of an exploding engine, asked herself what might be wrong. We now have a rather loud metallic vibration noise in the right rear side of the car which only happens under drivetrain load (I cannot reproduce this with revving the engine in neutral). It went away later that drive, but it now occurs on a daily basis and is certainly not normal. I can't believe others haven't noticed something similar at one point...

TheOtherEric

Alfa40

Update:

The bad: Noise #1 from my post above (the rattling when accelerating) was not pinned down during several visits to the mechanic.

The good: It has gotten more and more audible. It now (sometimes) also occurs when idling. This way, I hope I will be able to pinpoint it eventually.

The best: Today I turned off the engine rolling over a patch of bumpy road and with the engine off (both with engaged and disengaged clutch), the noise was audible. This should hopefully isolate it to something not engine or drive-train related.

Any pointers where metallic rattling could come from in the right rear of the car? It is much more audible from the inside as the engine noise doesn't allow me to pinpoint it from the outside.

Foxman

Heat shield?

Alfa40

Is there a diagram on which heat shield this might be? I had the car on a lift, took the large plastic engine panel out and my mechanic banged several parts with a rubber hammer to produce the rattle. We were not successful. I can't believe no one had this noise before, it is most audible when sitting in the car's right rear seats, it feels like it comes directly from where the seatbelt retracts into. Pretty sure it is not inside the cabin, though.

pp000830

Seat belt hardware or some other interior part rattling against some internal cabin surface?

TheOtherEric

It's super easy to remove the interior panels that cover the seat belt mechanism and the rear deck. Might be worth doing, doesn't take too long. Your mechanic doesn't seem to want to deal with this little niggle, and it's hard to blame him.

rk-d

Another possibility - a rattling worn out catalytic converter or loose baffles in the exhaust cans could make weird noises like this.