Looking at a 1988 Volvo 240 with 350,000 miles on it but the sound from the engine has me worried. Here's a video of it running.

 

check the valve clearances, also do a compression test. if those are good, then don't sweat it.

 

Boy, does that sound like a Rod knock to me! The "old School" way of finding out which cylinder it is, is to pull the spark plug wire one at a time when it's running till it's a little more quiet. That will tell you which one is the worst of the bunch.

 

Our 740 has a similar wrist pin noise but much lower in sound. Picked it up around 200K miles but it isn't a problem. Now have 370K and still running OK. It seemed to be a typical problem with these red block engines. I'll fix it when we restore the car in the near future.