https://www.youtube.com/watch?v=SLqHVCb7D2s  Right at the beginning, when the light first comes on, you can hear a VERY LOUD *BUZZ BUZZ* and then it hums fairly quietly as it warms up.  But what is causing that very loud buzzing right as it turns on.  It does this every night

Ther was a bucket that did that in my neighborhood and it got replaced. I'd think the windings in the ballast got loose and/or the ballast bracket is loose.

I guess you mean the short buzz just when the lamp ignites.
It is briefly asymmetric (rectifying) lamp on a HX ballast, leading to core saturation and so high current spikes.
That effect is normal, but usually it takes for shorter time, so yield just single "knock". Here the lamp just is rectifying somewhat longer...

Just at ignition, the lamp may briefly rectify, the resulting DC current bias then causes the ballast core to saturate, so the magnetic flux is pushed outside in way bigger extend than it is during the normal operation. And if there is something steel near by, it creates forces, so the noise.
The saturation happens for very brief time around the peak of the magnetic flux, so it's spectrum contain a lot of high order harmonics, which become well audible (together it forms the 60Hz "knocking" sound). After both electrodes warm up (0.5..1s after ignition) the lamp stops rectifying, so the DC current leading to that sound disappears.

The pulses are only narrow, because during the saturation the current gets very high, so causes rather high voltage drop across the winding wire resistance and that drop compensates the rectification and so prevents further DC shift in the magnetic field.

This effect needs a ballast with the secondary conductive for a DC current, so only HX or series choke ballasts. The CWA will never exhibit this, as the series capacitor is preventing any DC current buildup, with whatever rectification on the lamp.

Thanks for your replies.  As much as I hate to do this, i may get my neighbor to call Ameren and have them put a new HPS luminaire in its place.  The  is pretty dim anyway for a supposed 250 watts.  Hopefully he will put a cobra head in its place

I'd be leaning to let the Merc stay, but if the company is up to spot replacements, better get it replaced with HPS than wait a few years and get a LED.....

I would not mind using a low power (~70W..100W) HPS on my home or shed if i need the light

I'd be leaning to let the Merc stay, but if the company is up to spot replacements, better get it replaced with HPS than wait a few years and get a LED.....

I would not mind using a low power (~70W..100W) HPS on my home or shed if i need the light

LED is the last thing I want to see in my town, but I am pretty sure HPS is here to stay in the neighborhood.  They may put LED on the highway though

The only good thing about the changeover to LEDisease is the amount of Mercury vapor lights and fixtures you'll stumble upon on curbs and habitat due to their "obsolescence". Here's a good comparison between street lights and certain types of footwear:
Cooper ovx and similar sizes: essentially steel toed boots.
GE m250r2 w/o drop lens: tennis shoe.
Modern LEDisease lights: flip flops. And the LEDisease lights are of potato quality in real life, just like flip flops. The m250r2 varies: some are better than others, like competing brands of tennis shoes. The "steel toed boots" aka Cooper ovx and m400s will take a beating and do great in service for many years.
@M250R201SA: I have never heard of an 250w bucket. If you can get it from the linemen, I'd gladly take it, 250w buckets are somewhat elusive.

Have to agree. Most of my latest and greatest finds were taken out of their original locations due to LEDs. But better for all the good lighting to stay in service then to have 99% of it trashed and 1% become mine

The only good thing about the changeover to LEDisease is the amount of Mercury vapor lights and fixtures you'll stumble upon on curbs and habitat due to their "obsolescence". Here's a good comparison between street lights and certain types of footwear:
Cooper ovx and similar sizes: essentially steel toed boots.
GE m250r2 w/o drop lens: tennis shoe.
Modern LEDisease lights: flip flops. And the LEDisease lights are of potato quality in real life, just like flip flops. The m250r2 varies: some are better than others, like competing brands of tennis shoes. The "steel toed boots" aka Cooper ovx and m400s will take a beating and do great in service for many years.
@M250R201SA: I have never heard of an 250w bucket. If you can get it from the linemen, I'd gladly take it, 250w buckets are somewhat elusive.

The only 250W Bucket I had I already sent to Joe Maurath.  Here is a photo of it.  If I do get more though, I will certainly keep you in mind as first-in-line.  There are 2 in my town that I am aware of, so when the lineman changes them, I will try to see if he will hold them for me.  He only lives about 5 streets down from me.  Keep in mind, this is the same lineman that told me that he can no longer give me old stuff, however, everybody likes money, so hopefully, he and I can make some kind of arrangement.  Maybe if I give him a 100W Bucket to put in its place, he will give me the 250W Merc Buckets.

Thanks. Do they still have refractors? The refractors they need are glass I was told, so that's be hard to find.

I seen GE 250 watt SA201 mercury and they had arcrillic refractor on them.

So to keep up your shoe anology filament lamps are like my favorite comfy boots and preheat flourescents like my other faverout pink stilletoes

I seen GE 250 watt SA201 mercury and they had arcrillic refractor on them.

Acryllic Refractors are ok for 250 watt Mercury, so long as it is open-type fixture (the heat can vent)