I own a 406 hdi. definatley bottom pully. They have really comedown inprice lately too. used to be a main dealer only part. You can get them anywhere now

They are a cushion drive type thing and it fails. Have a look for tiny metal shards around the pull area and on the block.

I replaced mine with a 2006 spec item and it looks different and i beleive is a solid item rather than what used to be on it