1982 300sd, noise under hood when over 55mph
 

Many folks here have helped me fix all sorts of issues (minor), most recently knowing to replace the voltage regulator when i thought I made need to change out the alternator. Thanks!

On my way home from work last night when I got up over 55 or 60, this fast thumping, flapping, rumbling noise started coming from under the hood (certainly sounds that way). I'm running on a two tank WVO, but fuel selection makes no difference in noise. Accelerating or coasting make no difference in the sound/frequency of the noise, but the sound stops when the speed drops below 55. I put the car in neutral while the sound was going, and it made no difference. When I stop to look under the hood the sound is obviously done, and it doesn't happen when revving the engine in neutral.

My boss thought it might be related to the radiator fan clutch. Belts looks good.

In the last week, I put in a new stereo and cd changer (last monday)- then had problems with the car battery (last monday afternoon, put in the new regulator on friday), and then this. I don't think they're related , but thought I should put it in here.

Any thoughts from anyone could be helpful. I'm thinking of bringing it by Stephens Service Center (Sacramento) in the morning because it makes me nervous.

did you, change all the fuel hoses? including the cigar hose? i removed my cigar hose and put a non bulged hose and i had a thumping sound.

the cigar hose is the fuel line coming off the spin on filter to the return line, looks like its about to explode due to the bulge it has , which it is supposed to have

Since the sound is related to vehicle speed and not engine speed, you can eliminate everything concerning the engine.

Therefore, you're left with one of the two front wheels. Although your description does not appear to be a bearing noise at first observation, I'd pull the wheels and repack the bearings..........if they haven't been done in many years they are due anyway.

also if you have ATE calipers check and make sure the spring clips have the rods above the clips and not the clip just sitting there.. chilling with the rods not holding the clip down

Is your hood pad secure? It can flutter if it is starting to come loose.

is your air cleaner secure?
is the top tighly secured with the nut?

is your radiator and condensor firmly attached to the car?

you are looking for something that can move around at high wind speed.

Something else to eliminate: I had a similar noise that sounded like heavy rain on a tin roof that drove me bonkers. At 50+ mph a small strip of the windshield gasket would pop out from behind the metal frame (like a strip of rubber band) and bang against the sheet metal. When you stopped it would neatly retract.

Thanks for all the tips guys! Thankfully, this one was just stupid. The sunroof visor was catching wind at high speeds and bouncing up and down on the roof.

I'm so glad that's all this was, because I don't have the time or money to fix something more intense :)