what is growling/rattling/spinning behind my 300TD dashboard?

[240dddd]

At idle in my OM603 powered 1987 300TD there is a loud interior rattle that sounds like it is coming from behind the driver side dash (It's definitely driver side and not the interior temp fan thing that is behind the passenger side dash, i checked).

When applying throttle in park or neutral the noise becomes less severe, and when driving it isn't noticeable. When I shut the engine down the noise continues very briefly past the point of the engine being off. Sometimes it almost sounds like a noisy fan spinning at low speed.

I've tried locating the sound from the engine bay side but haven't been able to find it. If i put my ear up close to the brake booster I can hear it faintly, but I don't think a failing brake booster makes a noisy rattle.

I also thought that maybe it was the speedo, but the noise is worse when not moving so I doubt that is the case.

I plan to get into it at some point soon by removing the gauge cluster and possibly the lower panel, but I was hoping someone might already know what this could be.