Sorry guys, i'm new to this.
I am the original owner, have changed the oil and filter every 2500-3000 miles. No serious modification other than pipes and jetting of the carb I have installed HD oil filters at every oil change. I have checked the oil screen, and had a very little junk in it. I have used Castrol GTX 20w50 oil all the time except for once I actually used HD 20w50 oil. I have taken the bike into HD for the 10,000, 20,000 and 30,000 mile services. I was on my way to take the bike in for the 40,000 service, when this noise appeared.
Being a retired auto mechanic, and new to strange noises in the bike, as it's been great to me. Actually this is my first problem with the bike, and don't want to bring it to the HD dealer. I have the service manual on the bike, and I am willing to tackle the replacement of the lifters.
Thank you for all the replies