96 Outback - Loud knock on cold start

[Surlycamera]

96 Legacy Outback Wagon (BG) EJ2.5 159,000mi. all stock -
When I start my Outback when the outside temp is low it has what I have been thinking was an exhaust leak. Once it warms up it goes away in about 5 seconds. From really loud to no problem. It has not seemed to effect anything else. Drives fine, except if I drive it before the knock goes away it runs like crap. Hesitates, no power for about a block and then it's fine. There was a day that was cold 30-40 deg. and we had a warm snap and the problem went away. The weather got cold again, knock. I got the three exhaust gaskets and crawled under her tonight. I don't see any signs of the exhaust leaking but the heat shields on the manifold are loose. If I bang on the heat shield with my hand it sounds just like the knock I hear when it's cold. So I'm wondering should I still pull the manifold and change the gaskets? Perhaps the manifold has a crack that only opens up when its cold? If there is no leak I could see the heat shields banging and making the noise but it seems like it would have to be vibrating bad for that to happen. There is no noticeable vibration. It's been doing this for a while. Today my check engine light came on. Could be related, maybe not I have not had an opportunity to check the code.

?

[Surlycamera]

I pulled the exhaust manifold off. I didn't see anything to indicate a leak at any of the gasket surfaces. I checked the flanges with a straight edge and they are flat. The manifold to cat gasket I got is wrong so if I can get another gasket tomorrow, I'll put it back together and see if it's better.
I need to figure out how to keep the heat shields from rattling around before I put it together.

[r3v_v3ng3]

Maybe a bad oil filter. Something about a drain back :smt017

[Navi271]

Definitely check that CEL. But it just sounds like piston slap. I just hit 150,000 on my car a month ago and I've noticed my car is a little louder on startup than it usually was. You just gotta baby it and let it warm up before you do any hard driving. The piston rings gotta heat up and expand since they're old. Once they expand you'll have full compression and it'll run fine.

[mlitz]

Yup sounds like piston slap, my car sounds like it is going to fall apart in the mornings but once it warms up it goes away and it's back to WOTing.
(WOTing - the act of fully depressing the throttle)

[Surlycamera]

Wow, I had read about piston slap and I'm familiar with thei concept but I didn't think that was it. Maybe. I usually let my stuff warm up before driving.
I bought a code reader so I can hopefully take care of the CEL. I still have the exhaust off. One of the gaskets was backordered. Anyway, I'm going to figure out how to keep the heat shields from flappin'. I'll get the pipe back on this week. The weather turned hot here so it will tough to say since it doesn't do it unless the outside air is cold.
Sorry this is taking me a while. I had some family issues, and both of the other cars were broken so the Outback had to sit.

[Navi271]

Don't worry about it. We all, well most of us, have lives outside of our subarus lol. Just update us when you have somthing to update us with!

[Huffer]

Metal straps are good for the heat shields. Dealer's use them all the time for quick fixes on loose shields.

[Surlycamera]

O.K., so over the weekend I got the Outback together. As far as the heat shields are concerned, I welded them to each other and the exhaust pipe itself. The shields had some sort of heat blanket stuff on the inside at some point. That stuff is all gone and the fasteners are rusted allowing the sheet metal to flop. I just put a few tacks on the pipe holding all the pieces from moving. One shield had a crack so I touched that up as well. It went back on just fine. NIce and quiet. Much better than before. I can hear some piston slap but it's nowhere near the noise the shields were making. The slap goes away quickly but it was in the 90's all weekend so it's not like what I might hear in the winter. I also changed the oil.

Now the bad news...the CEL was a PO0740 Torque converter clutch sys. electrical.
After a bit of Google-ing I see that this can range from minor to pretty severe. The fluid level was a little low so I topped it off and cleared the code. I drove it a few miles and it came back on. I need to do some fiddling and figure out if its a simple fix or a death sentence.

Anyway thanks for the responses. The heat shield thing was a simple fix.