2000 V70 SE, 267K miles, B5244S NA w/VVT, Denso, AW-55

It's not really a rattle; it's probably best described as a squirrel cage or hamster wheel slight knocking sound emanating from the middle dash area.

Occurs continuously when operating the car. Shut engine off with key remaining in "I" position and the sound continues. I thought it might be related to the HVAC blower fan, but the knocking sound continues when fan is selected on, and when fan is selected off.

Turn the key from "I" to "0" and the sound stops. Of note, turning the key back to "I" after selecting "0" does not bring the sound back. The sound only repeats once engine is started again.

Otherwise, everything is normal. Car drives OK. All HVAC functions work, no flashing Orange warning light, etc. All instrument gauges and lights work, too.

So, I'm baffled! Any ideas?

Thanks Klaus,

That explanation sounds quite reasonable since the car is parked among pine and oak trees. And I'm continually cleaning (flushing) the two prone-to-clogging drain hoses in each corner of the cowl.

When there's an opportunity, will pull the blower fan and take a look. Meanwhile, I don't think there will be any damage done while continuing to operate.

No need to pull the blower. Remove the 3 torx screws holding down the plastic under the windshield and pull the cabin filter out to take a look for pine needles.

Is the engine warm when this happens? Starting in 1999, the blower does not stop when you turn the engine off( key moved from II to I). You might have a twig or long needle pine that has fallen down to the blower.
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Keeping it running is better than buying new