I just installed a new photocell on the mercury vapor area light that's been on the side of my house for years, however, I noticed today walking around that the light was making a slight buzzing sound when it was off during the day. I placed a bucket over the photocell and the light turned on and the slight buzzing stopped and when I removed the bucket the light went off and the buzzing returned, so it only happens when the light is off. Does anyone know what may be causing this? I've never had this happen before and I'm just curious.

What photocell is it?
If itis some older design without time delay, these were made as a simple electromagnetic relay with NC contact ad the coil in series with the CdS photoresistor, fed directly from the mains.
Then when there is light, the photoresistor passes current to the relay coil, it activates and opens the contact, and the AC current feeding the relay electromagnet may be buzzing (it depends on the exact relay design and how wel aligned the assembly is, or whether there is some dirt preventing the yoke getting fully uniformly attracted to the core so the phase shift "antibuzzing" shaded pole is able to keep it firm ON so it can not buzz, or there is agap, allowing some vibrations and hence the buzz)

When I was growing up, we had a HPS fixture on the shed. It had a used Fischer Pierce photocell that was instant action and it would hum during the day. I always thought it was to keep the capacitor charged to ensure adequate ignition at dusk. I later learned that it worked just like Medved said. If you turned off the power, you could hear it click to the on position. Upon powering it up again, a split second before the relay engaged, you could see a brief flash in the arc tube. It eventually failed when the window glue let go and it stopped working.

What photocell is it?
If itis some older design without time delay, these were made as a simple electromagnetic relay with NC contact ad the coil in series with the CdS photoresistor, fed directly from the mains.
Then when there is light, the photoresistor passes current to the relay coil, it activates and opens the contact, and the AC current feeding the relay electromagnet may be buzzing (it depends on the exact relay design and how wel aligned the assembly is, or whether there is some dirt preventing the yoke getting fully uniformly attracted to the core so the phase shift "antibuzzing" shaded pole is able to keep it firm ON so it can not buzz, or there is agap, allowing some vibrations and hence the buzz)

It's an Intermatic silicone photocell. It was brand new from HD so it's not an older one. I was just curious what it may have been because the ballast in that light makes a slight higher pitch buzz similar to what I heard from the photocell which is why I tested to see if the light would come on because it sounded like the light was dayburning but was unable to start the lamp, so I was worried that the ballast or lamp was shot. I appreciate your explanation!