Get a Harbor Freight stethoscope and check everything being driven by a fan belt.
It could be early bearing failure in any one of a number of accessory and idler pulley components. You can also pressurize various vacuum hoses to a couple psi separately and specifically with a regulator and blow gun with tapered rubber nozzles also from Harbor Freight. This is just a bit different than the smoke test but more of a Hiss test. Some internals of various vacuum devices could be failing and you wouldn't see it via a smoke test.

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2001 x5 3.0 (sold with broken motor), 1990 e30 M3, 1991 318is, 2002 325i, 2008 335i Dinan stage 3 6 speed,