Well, if it's piston slap, it has been gradually getting noisier. That would be the same kind of noise you'd get if you knock a wrench handle on the block--a kind of muted knocking. Naturally a rod knock is worse when warm, while piston slap sometimes disappears entirely.

Another likely possibility is that one of the valve absorbers died a horrible death. They have a finite lifespan, and when they go bad, they can cause a loud rapping noise as well. Fixing those is not an issue of rebuilding either. You just need to pull the camshaft out of the car and carefully remove each tappet out of the head (one by one in and out) and replace the absorbers. Sometimes it's miraculous, how quiet it makes the engine.

You should at least spec the valves and see if they're in adjustment. If something has suddenly gotten noisy, I'd be suspicious. But if this was somethign that has gotten worse gradually, I wouldn't be quite as worried. I have a 240 with engine noise (ticking) that has not gotten worse or better in 90,000 miles. Some would say there's a problem, but I've tried every possible fix save for throwing the engine out. It specs excellently--compression, power, everything. It's just a noisy engine, and that's that. Since it's the same as 90k ago, I'm no longer worried. I drive at 90mph all the time and it never gets worse, better, or anywhere in between.

I wandered away from the question. If you pull the head and have it gone over, you might find that the noise goes away. In that case it was obviously valve noise. You have to know if it's a lower end noise like piston slap. If that's what it is, you can visibly move the piston around in the cylinder. When you do, it makes the same noise as it makes when the car runs. The degree to which you can move the piston around is almost alarming, but it's mostly normal, as much as it hurts to acknowledge that.

If the problem cropped up suddenly under identical conditions to which you've been driving in the past (including ambient temperature) then I might start to worry about it. But if it's been gradual, then it's less alarming. Is the oil clean? No concerning metallic debris (other than the very fine stuff you can usually see)?

If--for example--someone ran the engine out of oil previously, then it could be a serious problem. But the 93 B230 is one of the best engines ever... it's hard to think it's got a serious problem other than previous owner's neglect or lack of knowledge, or a combination of both.

SOrry to drivel on and on about this. There must be a reasonable explanation, and you can confirm things before you go and spend money (perhaps needlessly) on an engine overhaul.
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chris herbst, five volvos.