We'll...my hunch was correct: dirty oil! A combination of the extreme
cold weather and my short commutes to work and back must turned the oil
in my 90, which was 3400 miles old, into manure. I have not heard even a
peep from the motor since I changed the oil and filter. I presume the
sound was a cam bearing not getting enough lubrication during warm-up.
I usually change both the oil and filter every 3000 miles, but the cold
weather and my lack of free time lately took their toll...