Well this is even stranger now, went for a ride to try to listen and warm it up and no racket. Rode about 40 miles no radio listening close, just before I got back it started again in the same rpm range. Whatever it is it is inside engine vibration and sound increase with throttle. Parked untill sat will pull cam cover then and peek inside. Hope for the best.
I'm not sure when the rattles started, but it seemed to be more pronounced on the '07 110" and later engines. I had an '08 SERK and it too had the rattle between 2500 to 3500, give or take a couple hundred rpms. I now have an '09 SERG 110" and guess what, same rattle, same rpm range. I think if you go back to some of the rattle threads for this specific rattle, they are all in the same rpm range, and there are plenty of complaints.
When I was in Sturgis '08, (Rapid City HD Civic Center) I talked to the tech guy at the parts desk about the rattle in my '08 110". He knew what I was talking about and said that it's harmonics in the valve train. Kind of like a loosely tuned tunning fork that get louder at certain vibration frequencies. He said that the beehive valve springs would be where he would start if it were him.
Has anyone used the beehive springs and if so, did they help to quiet down the rattles?
I think that when people here have changed out their cams, lifters etc, that the noise in some has pretty much gone away. This would lead me to harmonics as well. Changing lift and duration will change vibration frequencies within the valve train.
I don't disagree with piston slap either, but from my old car days experience with cracked pistons or piston slap, I think that piston slap is a more pronounced knocking sound versus our 2500/3500 rpm rattles.
I'm just a layman and without any HD experience to speak of like the engine builders on our forum, and they know more about this than I do. But it sure seems to me that with all the years that HD has made engines that they should by now be able to make one so quiet that you can't hear it running, at least from a motor noise standpoint. This is one area that a true waterpumper would help insulate the rattles from us.
Good luck with your rattles and it sounds like you have the same rattle that most all of us has experienced with the 110" engines, but my noise is always there, it never goes away like you say yours does. So you "may" have something defined causing your problem. With what I've read here and in other places, the problems with the stock lifters is enough to make me want to replace them before they fail. Once in the cam chest as others have suggested, change them out with the cam bearings.