Noises that change with steering or gear
Here's one. You turn the wheel and a rumble, a growl, gets louder. Or it shows up when you drop the drive into gear and goes quiet in neutral. That's the driveline talking, somewhere between engine and drive. On a sterndrive, look at the gimbal bearing first, then the universal joints in the drive shaft, then the engine coupler. What gives it away is that it tracks your steering or your gear, never your revs. Don't shrug this one off. When a gimbal bearing goes, it takes the drive shaft and the bellows along for the ride.
Urgent? Get-home urgent. Not kill-it-this-second urgent. Go easy, keep the turns gentle at speed, and have the drive pulled and checked before the next trip out. If you've got that growl and there's water in the bilge under the engine too, you're done for the day — a failed bellows behind a failed bearing is exactly how boats end up sunk at the mooring. Steering noise that isn't the driveline is usually the steering system itself, and that has its own page, so it's not repeated here.
Shift, gear and drive noises
Grinding, or a big clunk when you shift? Usually the shift mechanism isn't traveling cleanly. Start with the cable adjustment. When it's off, the clutch dog only half engages and gets its edges chewed every single shift, and worn dogs bring a new trick: the drive jumps out of gear under load. Both live in lower-unit territory, covered properly elsewhere. Short version: a rough shift is a problem today and a dead gearcase later.
A whine that climbs smoothly with the revs is another story. Changes with engine speed whatever gear you're in? Probably something bolted to the engine — the alternator or another belt-driven accessory, a pulley bearing, or on a supercharged engine the blower itself. Only in gear, and it follows boat speed instead? Gears, inside the drive. A bit of gear whine is normal in reverse and at some speeds. A new whine, or one that gets louder as the summer goes on, means pulling the gear oil and having a look.
Vibration that appears at speed
Smooth at low speed, shaking once you pass a certain number? That's almost always something spinning: the prop, the shaft, the drive. A nicked or bent blade will do it. So will a spun rubber hub, a bent shaft, a prop out of balance — same signature every time. The first check costs nothing. Get her on the trailer and actually look at the prop, leading edges and hub included. (Props have their own page, so the detail isn't repeated here.)
You can't run a vibration out. At speed it leans on bearings and seals that weren't built for it, and give it a season and a small imbalance becomes a gearcase bearing failure. Shaking starts halfway through an outing? Ease off until it stops, then run home under that speed. Around here, plenty of speed vibrations begin with a rock or a sandbar close to shore — the narrow passages and island channels especially, where the bottom comes up fast.
Knocks, squeals and rattles
The knock. It shows up under load, goes away at idle, and it's the one to take most seriously. Could be detonation, from poor or stale fuel, timing that's off, or overheating. Could be a bearing on its way out. Detonation chews up pistons fast; a bearing knock finishes the engine. Either way: back off the load right away, idle home, and don't start it again until somebody's found the cause. Of every noise on this page, this is where pushing on costs you the most.
Squeal on a cold, damp morning that fades after a few seconds? Usually a belt slipping, and mostly harmless — right up until it stops fading. Then it's the belt or its tensioner. A rattle at idle that disappears with revs tends to be a heat shield on the exhaust, a cowling latch, a bracket, a mount that's loosened up. Harmless on its own. The loose mount is still worth finding, though, and a rattle is much easier to chase while it's only a rattle.
Alarms, cooling and the general rule
Alarms aren't noises you interpret. They're orders. Overheat, low oil pressure: you shut down, you don't slow down, and you do it now, however sweet the engine sounds. Cooling trouble usually shows itself through the alarm and the telltale stream, not some mechanical sound, and there's a page here for impellers, thermostats and the rest. Alarm goes off and you can't see why straight away? Engine off, get a tow.
One rule beats this whole index. A new noise is free information, and the first time you hear it is when it costs the least to chase. Noises almost never fade away on their own; they get louder and drag something else down with them. With a New England season this short, a breakdown in July eats your summer, while that same fault caught as a funny sound on the first run after ice-out is usually a small job. Make a note of what it does and when, and get it looked at.
