340 noise issue
Add a bottle of marvel mystery oil before changing the oil ,let it run about 15 minutes or so varying the engine speed.
Dump the oil and replace with Mobil-1 15w50,it has all the good stuff flat tappets need to live, just check their web site for the nay sayers.
Had numerous small blocks over the years and currently own a 340 I originally built in 1984.
If you let it sit over two weeks it would get a lifter noise at first start up that would go away in a few seconds no matter what oil was in it.
The worst was Castrol syntac,
After switching to the mobil-1 the lifters stayed pumped up no matter how long it sits
Been using the Mobil-1 in this 340 for over 15 years and recently pulled the engine apart for a head and cam upgrade and the cam looked excellent with no noticeable signs of wear