Just bought an older house and I was standing next to the panel when the AC kicked on.  Breaker buzzed for about 1/4 second then everything was OK.  Hung around a bit and it does this every time the AC kicks on.  Normal?  Need to replace breaker?  Something more serious?  It is a 50A/2-pole/Type C Challenger Electric breaker.

Main buzzed or a/c breaker?

I'd swap it out and see if that fixes it.  But I'm not an electrician.

If it's a square D QO panel, the breaker might be loose on the bus bar. Breaker replacement. Just my two cents...

replace breaker,
it's easy as pie.

Just cut the Main off
before you start.

get a circuit tester and troubleshoot the wiring if the breaker doesn't fix it

It is the AC breaker, and it is a Challenger panel.  I just read an article on the net that said Challenger panels can catch fire.  The home inspector passed the panel. I just pulled the breaker and there is no arcing or discoloration on the buss, and the breaker is not melted (classic sign of a bad Challenger breaker according to the article).  I guess I'll just swap out the breaker.

That whole panel needs to go.  They are in the same class as Federal pacific.

The home inspector passed it. Lol.
That just means he passed by it on the walk thru.

I have a 200A square D QO that buzzes when my ac comes on. It mostly sounds intrenal from what I can tell. It has done it since it was new and doesn't appear loose or anything. It's actually bolted to the buss. It's 10 years old and never has tripped or gotten warm , nothing.
I thought about replacing it but seems to work fine except for the short 3 second buzz / vibration or whatever it's doing.

No, he took the cover off and checked for issues.  Found one cable with no clamp and wrote it up.  Will likely change the breaker but not the panel.

Challenger hasn't been around for quite a few years. That said, circuit breakers are not supposed to buzz. Replace it. A 30A 2-pole (I assume OP is talking about his condenser) are cheap...

I am a master electrician. I would suggest that you have a qualified electrician inspect your AC unit and panel for loose connections. Your panel is not the highest quality!

That said, when an AC unit kicks on, it will draw well above it's normal running current rating. This is usually at least 125% of the normal running current. In some cases, it can be higher. You will find two amperage ratings on your AC unit. One is minimum circuit ampacity(wire size is determined by this) and over-current protection(breaker size is determined by this). HCAR breakers are rated for these loads on start up. Your breaker may not be HCAR rated, may be loose on it's connection to the buss bars, or just plain bad.

Again, call a qualified, licensed electrician. Your electrical system is the one thing in your home that can kill someone or cause a fire. In old homes, if the wiring is rubber insulated, it is very possible that the insulation has broken down from age and overloads. Most older homes were never wired for all of the electrical devices that are available today. They had lighting and outlets that were intended for fans, radios, or the occasional vacuum sweeper. AC was never taken into consideration!

I have worked on homes that were built before they were wired using old knob and tube exposed conductors in the basements and attics. More likely yours has cloth covered Romex which is nearly as bad. Great care must be taken when working on these systems!

Qualified electrician!!!!

House built in 1989.  Standard Romex.  I tightened all of the hot leads but not the neutrals.  Breaker is tight on the buss, no arcing or discoloration.  42 slots, 34 used.  A couple of HCAR breakers in the panel but this is not one of them.  House was built by a lineman for Entergy.