30 second rattle, carbon chunk ?

[blazer2007]

1980 2 wheel short box,stock 350/350. Rebuilt about 6 yrs. ago,only driven in the summer about 4-5 months of the year. Hardly ever on the hi-way which could give me carbon build up,mostly 30 mph.Small town of 60,000 pop. Anyway was driving home and suddenly heard a rattle ticking noise,a noise that you know is not good or normal,pulled over gave it a little rev. and the noise went away. All good for the ride home.Took valve covers of and nice and clean,nothing missing on the valve train. I never did get around to putting the inspection cover back on, wondering about that,will inspect that but was thinking that would make a different sound. What do think,have you had something like this happen to you.