I've had that same issue for many many years. In my case, and probably most others, it wasn't anything serious. It drove me crazy for a long time because I thought it was going to kill me. I went to three different cardiologists who all said there was nothing wrong. If you've been to doctors you trust, you can probably rest easily. As for what causes it, I don't think anyone knows for sure. Strangely, for me, it happens to me far less as my fitness improves. By the middle of the season it's gone almost completely. Then it gradually comes back as my fitness drops. I guess it's just mother nature telling me to get my ass on the bike!