loud noise on cold start

Sounds like you may not be getting oil or enough oil at startup, you will need to have the oul pressure tested.

mechanic doesn't think it has anything to do with oil or oil pump. Have to bring it back next week when they have more time to look at it.

The timing chain may be loose. What weight oil do you use?

Whatever my mechanic puts in. It's synthetic.

I would try going up one oil weight level and see if anything changes.

They're all wrong. It's simply piston slap,which will lessen upon warmup. Stick with 10w30- 40 dino.

If you have a Subaru with the PZEV badge this is completely normal. It's a function of the PZE system. It idles the engine at a higher RPM (usually between 1500 and 2000 RPMs) when it first starts until the oil has reached temp, you accelerate, or shift gears. I guess they find the engine performs better when it gets to temp quicker. It scared me too but brought it into Subaru and they showed me all of them do it from the factory and SOA confirmed it as well.

This gets a little tricky, as the newer DOHC 2.5i w/ chain-driven cams are quite clacky until warmup. Further, there's an annoying incidence of cracks in the front exhaust system that cause quite the cold roar, sometimes sealing up to near-normal noise level once the pipes expand and close up the crack(s). Fortunately the latter is covered by extended Fed-spec emissions warranty, as cutting out and re-welding a new flex-pipe module is time consuming.