I would look at the flex plate.
I had an 86 with a 305 that did the same thing about every 40K miles. It was on its 3rd flex plate at 124K miles when I sold it. Each time the symptoms were the same. In park it would clatter like a cheap dinner bell but in gear or at increased RPM it was silent, and each time I found the flex plate had cracks between the crankshaft bolts.
Is it gradually getting worse? Mine did. If it is I'd suggest checking it quickly so the converter doesn't wobble around and damage the front pump and send trash through the transmission.
It's a pain but not a huge deal to fix. As to why the bolts would be breaking? I suspected crank walk with my 305 but didn't care to dive in because I wasn't in a place where I could do anything about it.
You might find something simple like a loose converter bolt. If so then it's a quick fix.
I wonder if you could pull the dust cover and put a cheap bore scope camera up there and look around? eBay has them for phones and laptops for about 5-10 bucks.
https://www.ebay.com/itm/7mm-Android...ZIWRIhYr9H2fqg