Any guidance welcome.

I work in a private customer friendly
and honest and fair shop. I work with
two certified mechanics. A customer
came in with a 2012 Silverado 5.3
with 90,000 miles. It was making a
ticking thrashing sound like a card in
the spokes of a bike but loud. She
said the noise started suddenly
accelerating onto a freeway. But now
the sound only starts after warm up.
At first we thought the harmonic
balancer may have separated. So we
remove the belt and started the
engine no change. We put it on the
Hoist and put the stethoscope on it.
On the engine block top end and
bottom end nothing. But on the
bellhousing between the starter and
bottom inspection hole it was loud.
So we remove the transmission flex
plate was in good shape. So we put a
new torque converter in it. The sound
was slower to start and not as loud
but still there. So we took it to
another mechanic who specializes in
Chevy trucks. In fact it's all he works
on. He seemed to think it was a
stuck lifter. But had some of his
mechanic friends listen to it and they
seem to think it was a cracked
flywheel or torque converter which
we know it's not. So he took the valve
covers off to look at the Rockers and
couldn't find anything out of the
ordinary. And wanted a grand to redo
the heads. We are at a loss at what it
could be we don't want to start
throwing Parts at it. It runs great
shifts great just noisy any help or
advice would be appreciated thank
you so much to everyone.