Just finished a 2300 mile trip. Second last day started the bike in 50 degree weather from sitting all nite. Very load rapping noise for about 10 seconds coming from top of engine. Drove & the noise would come & go at highway speed (70). Sometimes very loud, other times nothing. Fresh oil & level was up. Next day, same thing. Sounds like oil not getting to top end quick enough. Just started bike after sitting a day & it rapped again for the first 3 seconds. Never did this before. It's a 13 with 18,000 miles running amsoil oil Should I be concerned??