I'm getting a weird slow frequency rumbling/rattling noise from my 96 Sportster with 38k miles on it. It started as a slight, barely audible noise, and gradually got louder over several weeks.

By slow frequency, I mean that the noise cycles a little faster than once a second (I would estimate 80-90 times per minute) at idle speed (about 1000 rpm). The noise is weird and hard to describe; it sounds kind of like a horse galloping on a hard packed dirt road.

The noise is the same cold as it is at running temperature. The frequency of the noise increases proportionately to increase in engine rpm, and gets a little louder at higher rpm.

Even with a stethoscope, I can't pin down where the noise is coming from. It seems to come from both top and bottom. I checked for any problems in the primary (clutch hub bearing, drive sprocket lock nut, etc) and the problem doesn't seem to be in the primary.

The frequency seems way to slow to be a valve train problem or a rod bearing. I was trying to think of anything that would produce a slow frequency noise like this and the only thing I can think of is the oil pump. With the gear reduction of the worm gear drive, I figure that the oil pump probably turns less than 100 rpm with the motor idling at 1k rpm. If that is the case, it matches up pretty close (or maybe exactly) with the frequency of the noise I'm hearing.

Anybody ever heard a noise like this? Any ideas? Any input is appreciated.

 

Just finished finding a noise in my 97 and fixing it, although it was nothing similar to yours. This crap will drive you crazy. Could it be something in the cam drive system? Loose engine mounts that a harmonic is affecting?

 

I think it might possibly be a cam lobe hitting the pinion shaft bearing race. Two things have lead me to believe that it probably has taller cams in it:

1. I originally thought the noise might be coming from one of the rocker boxes, and I replaced all four rocker arms. After that the new rockers started hitting on the rocker box cover. I ground them off a bit so they don't hit any more, and looking at the old rocker arms, they look like they were ground off a bit as well.

2. I pulled the lifters to see if a lifter roller was going bad, and found that it has aftermarket lifters in it that allow more clearance from the shoulder to the anti-rotation pins (to allow for higher lift).

These two things lead me to believe that the PO put taller cams in it, although the push rods and valve springs appear to be stock.

I was reading an article about lobe swing clearance issues with taller cams, and I started thinking that I might have a cam lobe hitting the pinion shaft bearing race. So I thought; why would the problem start this long after the cam change, and then I thought maybe the clearance was super tight, and a small amount of wear in a cam bushing allowed the cam lobe to start hitting.

But then there's the frequency issue. If a cam lobe was hitting, it should hit 500 times a minute if the engine is running at 1k rpm. But then I thought, maybe there is just enough end to end play that the cam is moving back and forth, hitting 3-4 times, then moving sideways and missing a few times, then moving back and hitting a few times, on and on in a continuing cycle.

I forgot to mention that I also pulled the cam cover to see if any cam bushings were causing noise. The only one that seemed like it might be excessively worn was the nose cone bushing (the #2 cam bushing in the cover). I had that bushing replaced at the dealership service dept, but it didn't make any change at all in the noise I'm getting.

 

Wish I could be of more help, but I'm not real sharp on Sportster engines...yet. Sounds like you may be on the right track though. Best of luck with it. Like I said, things like this drive me crazy till I find a definitive answer.