I suggest you start with the basics: change all the fluids to the following:
- FULL synthetic VTwin 20W50 engine oil
- FULL synthetic 75W140 GL-5 gear lube (tranny)
- Fit-for-purpose chaincase/wet clutch fluid in the Primary
- HD oil filter
See if that helps. If not, I would rule out exhaust leak by soaking studs/nuts with PB several times and then CAREFULLY snugging up the nuts a bit. BE CAREFULL doing this. You do not want to twist off the studs.
If that does not help, then you are probably gonna have to replace the lifters. But get a stethoscope and see if you can definitely identify one or more lifter making noise. If you have to do this, get some adjustable pushrods so you can cut the solids out rather than disturbing the top end.
TQ