Thumping when accelerating

[AcogDoc]

Well.... I’m back!! Boat is running took it out for the first time since the switch boxes were fixed. I have a 1996 mercury 150 off shore on the boat (2 stroke) and when I’m accelerating I get like a thump... thump... thump.. it’s pretty noticeable and you can feel it. If you accelerate quickly it will go away and you can slow back down and everything IS fine. But it’s always the initial acceleration. It’s not like grinding from putting in forward or reverse too slow it’s not like that at all. It’s a more like thump than a grind. Any ideas what this could be??