Weird noise!

[Dannym]

'87 E150 van 302 engine:
I was on the highway the other day for awhile and suddenly heard this... I dunno, flapping/rattling/scraping sound? Very difficult to describe but it was distinct when it kicked in. Not deafening loud but it's fairly alarming as noises go. It sounded to me like it was coming from the front driver's side wheel area. I guessed a seized wheel bearing.

I pulled off the highway are started looking for a place to park and check it out. I didn't find one at that exit, but while I was on the service road looking for the next exit it went away and was gone. I figured since it was a windy day, maybe I caught a plastic bag or something there and it was flapping madly in the wind.

Well, I was just out driving again, in town this time, and there it goes again. Engine shows 40 PSI oil. It's cold out, in the 50's. A/C is off. Traffic was heavy and I stayed light on the engine. I put in it neutral and coasted and the sound went away. Hit the gas when it's in neutral and the sound came up with engine RPM, IMHO it sounded weaker than the same RPM when driving the engine with throttle but it's hard to say. Left side of the vehicle for sure.

Then... within a few minutes it's gone again. No noise. Sounds fine to me. I didn't feel anything funny in the driving performance either time.

WTF could this be?? All I could match with the left side thing, not tied to the suspension, would be power steering pump (the noise didn't seem to react to steering input) and A/C compressor, maybe exhaust manifold. I had an exhaust manifold crack before that was loud when the engine was cold but as it warmed up the metal expanded and partially closed the crack. That wouldn't show up after driving awhile, make noise, then go away.

I seriously can't place this noise. Maybe water pump but the location sounds wrong. And I can't rule out the engine itself, it doesn't sound like it's coming from inside the engine but it's hard to say.

[jonesg]

I'd check the fan and the serp' belt to start with, no sense jumping to catastrophic causes yet. Could be something as simple as belt tensioner.

[rikfish]

I'd also check the fan clutch. First one I had go made intermittent sounds too, although I don't recall any 'flapping' noises. Have you noticed any loss of power when this occurs? If the clutch locks up at higher driving speeds you will probably notice a serious loss of power too.

[Dannym]

Hmmm well it's a thought. How would I even test that?