If the generator in question is an Onan MDJE 7.5kw, what you are hearing is abnormal. We have one of these generators on our boat---- it's been there forever, possibly since he boat was new---- and it starts smoothly and runs smoothly.
I should qualify "smoothly." It uses a two-cylinder engine that produces a lot of vibration. This is normal.
It can start hard and run rough for awhile IF insufficient time is given for the glow plugs and air heater coil to do their jobs. I was told when we bought the boat to hold the spring-loaded pre-heat switch on for about 5 seconds. This is what we did and the engine always started very rough and took a five or ten seconds to smooth out. Then I read the manual and found that the pre-heat switch should be held on for about 45 seconds under normal temperature conditions and a minute (but no more) in colder conditions. We started doing that and the generator starts quickly and idles smoothly (relatively speaking) as soon as the start switch is engaged.
The other thing that can make one of these generators run rough and idle poorly at startup is a failed glow-plug. One of ours failed and the generator became very rough starting no matter how long the pre-heat switch was held, which makes sense since one cylinder was not getting any heat at all. Changed the glow-plug and we were back to normal starting.
But rough running at startup and metallic or grinding sounds are two different things.* Even when the generator was starting rough, it didn't make any more noise than when it was running smoothly, it just ran rough.* Even before we knew the proper pre-heat time, or when the glow-plug failed, the engine did not sound like it was crushing or grinding anything, it just ran very rough until the cylinders heated up enough for proper combustion.
If the generator in question is not an MDJE than none of what I've written may apply. The MDJE and a Northern Lights are the only two marine generators I've had any experience with. The Northern Lights, by the way, is a whole different ballgame.
-- Edited by Marin on Thursday 3rd of December 2009 12:51:06 PM