I've got one and it's pretty noisy.
Bonding the tail-pipe to the body helped quite a bit. I ran a ground strap from the rear-most muffler hanger on the frame to a pipe clamp on the tail pipe itself.
I haven't had the motivation to really get under the hood so I have a fair amount of noise from various sources under there.
Good news, at least for mine, is that my fuel pump doesn't seem to be my trouble. It's also not noise on the electrical feed, it's only an issue through the antenna.
The noise level for me goes through the roof when it is wet outside. Either morning dew or rain will make things really noisy and negate anything but comms with S7+ stations if the engine is running.
For the 857 I put the body under the driver's seat and mounted the head in the change tray on the top of the dash using an outlet box cover plate that I just bent in a vise for the right angle and screwed the Yaesu mounting plate to that.
I used a ghetto solution of wall anchors through the dash to screw down the outlet box plate. I'd rather use pop rivets but didn't have a gun handy at the time.
Here's a few pics of what I didhttp://www.kingsqueak.org/photos/yaesu_857_jeep/
I wouldn't do the Larsen on that mount again, the ground isn't very good for it even though I've strapped it. That antenna wasn't the best choice for a mount like that. It works, but from what my analyzer tells me, it's far from optimum.