1998 Volvo S70 Terrible noise

[aaronius]

A head gasket leak on my 1998 Volvo S70 (5 cylinder automatic, non-turbo) caused the coolant to get into the oil, making a nice sludge. No oil droplets floating in the coolant tank, though it started to look grimey so exhaust gases may have been forced into the tank.

I stupidly drove it a couple times after changing the oil, and it suddenly developed this terrible engine noise while going about 50mph (80km/h). Note that unless the thermo is busted, it has never overheated to my knowledge...

Anyway, I pulled over, got it towed , did another oil change, checked and the noise is still there even merely idling after starting up (this is the chugging sound in my youtube video: )

I've since torn it down to the block to replace the gaskets , but the head and valves look fine (nothing obviously warped or bent). I was expecting to see something obvious causing the noise, so I haven't yet put everything back together.

Anyone have an idea as to what this noise might be?

Thanks!