Hmm.
So, my experience is that when a lifter goes it can do real damage to the 'well' it sits in as in break part of the well wall away. On mine, the lifter then 'floated' around and wore away the well wall so it became smooth. None of said wall could be found so we assume it's in the sump! This of itself isn't so bad as aluminium isn't likely to damage bores.
HOWEVER. When the wall breaks the oilway that feeds that well is suddenly becomes completely unrestricted. Oil pours out where it can go fastest and consequently lowers oil pressure to everything else under that rocker box. In my case, being starved of appropriate levels of oil did for my exhaust camshaft too.
As a fellow sufferer, I strongly urge you to take the rocker cover off asap and inspect the lifters and the wells they float in under the rocker arms. If a well wall has broken then it a 'new' head. When you reassemble, I'd strongly advise new lifters & rocker arms. Obvs you need gaskets, cam shaft oil seal and new timing belt kit (change idler arms too).
The best you can hope for is simply collapsed lifters. The worst is as described and you'll not know till you strip it.
If you need new lifters and rocker arms, I'll be selling my brand new (unused) set for £182 posted. Might even have the proper Volvo camshaft end seal too.
I'm keeping my fingers crossed yours isn't as bad as mine
