Any thoughts on troubleshooting my "mystery noise"?

[vetplus40]

I have a 2004 5.0 Mercruiser with Alpha One that has developed a mystery noise from the alternator side of the motor. With the engine compartment closed, the noise sounds like a classic ticking lifter. With the engine compartment open, it sounds more like an exhaust leak. The boat has aftermarket exhaust pipes on it, so the engine is rather loud sounding. I have done a few searches and so far have not turned up my type of problem. So far, I have done the following troubleshooting:
Checked oil level with fresh oil = level good.
Put mechanics scope all over valve cover and head = found no unusual noises.
Ran my hand over all exhaust ports that were accessible. No leaks found.
Ran engine briefly at night looking for stray spark = none found.
Taped exhaust flapper open to prevent rattling = no change noticed.

Based on recent searches, others have recommended checking fuel pump and injector, but I wanted to see if anyone has any other ideas. The noise is definitely not a "rattle" as the frequency of the noise directly correlates to engine rpm's.

I realize helping troubleshoot without any audible reference is not easy, but your comments are greatly appreciated nonetheless. Thanks.

 

[harringtondav]

I had a similar noise in my 4.3. Turned out to be a bad spark plug that was cracked at the insulator joint with the metal body. I didn't know what it was until the insulator blew out of the plug body.

 

[muc]

Short length of garden hose held close to one ear, ear plug in the other ear. Move the free end of the hose around near where you think th noise is coming from.

 

[H20Rat]

Remove all belts and fire it up, just to eliminate any accessories making the noise.

 

[kenny nunez]

To rule out any exhaust leak squirt some automatic transmission In the throttle body, if there is a leak you will see some smoke.
Also remove the valve cover and check for any loose rocker arms.

 

[Bondo]

Ayuh,..... Yer motor has an electric fuel pump, not mechanical, so it ain't that,......

 

[vetplus40]

I had a similar noise in my 4.3. Turned out to be a bad spark plug that was cracked at the insulator joint with the metal body. I didn't know what it was until the insulator blew out of the plug body.

Did you notice any performance issues with a defective spark plug? The last time that I had a cracked insulator, the motor lost fire in that particular cylinder and you certainly knew it. I have never had a noisy injector, but I guess it is possible.

 

[vetplus40]

Short length of garden hose held close to one ear, ear plug in the other ear. Move the free end of the hose around near where you think th noise is coming from.

I have a mechanics stethoscope. I went all over the valve cover and head. All I heard was nice quiet sounds.

 

[muc]

I have a mechanics stethoscope. I went all over the valve cover and head. All I heard was nice quiet sounds.

So do I. But the hose works better for some noises, especially a exhaust leak.

 

[harringtondav]

Did you notice any performance issues with a defective spark plug?

It started as a click as I was going from the ramp to our lift after the first splash of the year. No wake most of the way to our lift, so when I cleared the zone I powered up. Thinking it was a sticky lifter or something. No missing, just clicking.

Five miles up river I went WOT, thinking it may shake something loose. KaPow!, Then the click turned to a 'chuff-chuff". I idled home at 1400 rpm. chuff chuff.... Thought I'd blown a head gasket or cracked an exhaust manifold. No noticeable miss, but I was preoccupied with the thought of a head pull.

Back in the lift I noticed the insulator and a major carpet burn in the engine bay. What a relief. Figured sometime prior I'd cocked the spark plug socket and loosened the insulator.

 

[vetplus40]

So do I. But the hose works better for some noises, especially a exhaust leak.

I will definitely give it a try. Thanks.

 

[vetplus40]

It started as a click as I was going from the ramp to our lift after the first splash of the year. No wake most of the way to our lift, so when I cleared the zone I powered up. Thinking it was a sticky lifter or something. No missing, just clicking.

Five miles up river I went WOT, thinking it may shake something loose. KaPow!, Then the click turned to a 'chuff-chuff". I idled home at 1400 rpm. chuff chuff.... Thought I'd blown a head gasket or cracked an exhaust manifold. No noticeable miss, but I was preoccupied with the thought of a head pull.

Back in the lift I noticed the insulator and a major carpet burn in the engine bay. What a relief. Figured sometime prior I'd cocked the spark plug socket and loosened the insulator.

I will certainly check my plugs. I have had two instances of momentary sputtering like a bit of water in my fuel, but they didn't last very long. I am beginning to wonder if I have an injector going bad?