My grandparents just bought a house that has a cheap Regent merc light on the garage. The ballast buzzes really badly, it is loud and you can hear it vibrating from time to time.  The photocell that was in the fixture was dated November of 1993, so I am assuming that is also about how old the fixture is as well.  How long can a ballast continue to operate while buzzing? I didn't know if it was about on its way out or not.

The 175W Merc on our utility pole has buzzed as long as I can remember and it is still going strong after 35 years! It is operated dusk-to-dawn and supposedly has the same lamp in it that it has always had! It is definitely time to change the lamp, it is so dim it is almost useless. When I finally get around to changing it, I will see how old the lamp really is.

If you are interested, you might take a close look at the light and see if maybe there is something rattling inside the fixture that could be secured or see if the ballast is loose somehow.

-Nelson

Here is a link to a fixture that is making the exact same noise as theirs is. Is the fixture still safe to operate this way, and how much longer can I expect it to last? It is currently 17 years old and it is just the cheap Regent yard light.

http://www.youtube.com/watch?v=SD7nnroqpfw

If the ballast are so badly buzzing, replace it.
It seems that in the US, buzzing ballasts are more common then in Israel, since the ballasts in the US are mostly autotransformers and CWA and not chokes or reactors and in Israel, the majority of the noisy ballasts are from the 80' and earlier that are rare today and/or also CWA.

Unless the noise is annoying, I would not worry about it.  Check to see if it is securely mounted, as a loose ballast's vibration is transferred and magnified to the fixture housing.   It will probably run for many years while buzzing.

@DieselNut:
But i would be careful:
- Buzzing unusual for the ballast model may show developing fault
- Related vibration may shear off the winding insulation enamel, leading to short between turns of the wire. Such short locally overheat this turn, yielding to further insulation degradation. This defect spread then like an avalanche and end up with burning ballast with the risk of something else catching fire...

Buzzing is usually caused by loose laminations or windings. As it's been mentioned, it can just be a loose screw! It'd be worth checking the fixture out 

Hmmm... That is a bit loud. Like everyone else said, check for loose parts!

You could also apply class h transformer varnish. The windings may be worn.

Where can you get class h transformer varnish?

It pops up periodically on eBay in handy spray bottles or you can go to an electrical supply or motor store and they may have some you can buy.

Would this be the same thing?

http://www.sprayon.com/product-categories/electrical-and-electronic-cleaners-and-degreasers/clear-insulating-varnish-aerosol-el600

I've heard of members using that class f varnish and it works. Just use the ballast and once it warms the lamp to full, apply the varnish and the ballast will absorb the varnish applied.

Does one work better than the other? It seems like class H varnish is harder to come by though.

I'm not sure. You can use red varnish and I've heard it works like a charm.