'88 2.1L
Recently I've been hearing a hard thunk at startup.  Almost like
somebody is kicking her in the rear.  The thunk only happens the
second the engine catches.  No thunking while the starter is doing
it's thing.  After that, it purrs along just fine and no thunks occur
on acceleration.  It seems like this eliminates engine mounts.  More
prevalent after the van is warm like during quick trips to the
store.  I recently replaced the muffler & cat but I double checked
all the hardware and everything is tight.  Of course it doesn't make
a noise first thing in the morning.  Kind of stumped.  Of course my
worst fears are rods and bearings but it seems like I would hear
weird noise during normal driving.  Any thoughts?

Barrett
'88 Wolfsburg 2.1L