Thanks to the advice from posters so far, I am about 90% sure that I will have the 240 engine fixed unless it is some outrageous problem. I tried some Seafoam to see if it might be possible to unstick something in the valve train; the noise is a little quieter but still present (my wife thinks it's much quieter, but it sounds only a little different to me). Other than the noise it still runs like a top with plenty of vroom.
I am still suspecting either something with one or more valve adjusting shims (the valve clearances have not been been adjusted in 102,000 miles so those puppies have been in there a long time; I wonder if they are bouncing around on top of the cam followers) and/or the rubber "hushers" between the valve spring keepers and the cam followers.
On the advice of a friend and long-time 240 owner, I am going to see about taking the car to Whitney Scherer in Minneapolis for his opinion. Hopefully the problem is at the top of the engine and won't require the expense of yanking the head. It may need a new front seal as there is a slight oil leak up there somewhere.
Of the various things wrong there are some things I can readily do myself, such as checking the flame trap. Another project is that suddently the high beams don't work. Switching them on causes the headlights to turn off and the high beam indicator on the dash doesn't come on. The high beam relay makes a firm, audible "click." I have not yet done the obvious and looked at the bulbs themselves, although the bulb failure light does not come on either. I have not checked fuses yet. Are there known problems to look for? Do the high beam relays fail like the overdrive relays?