My 01 Fatboy with 18k has recently started making a rapping sound when I start it. It has happened maybe the last half dozen times I've started it. It will make this rapping sound 4 or 5 times right when it starts then quiet right down. And if I shut it off and wait a few minutes it will not do it. My guess is lifters bleeding off? Any opinions? And will this hurt anything if it is? I would prefer to fix it before it causes any bigger issues. Thanks.