I'm not sure what makes it happen, but it's not ALL the time. When playing an Engineer, you'll sometimes hear a continuous beeping sound.
Anybody know what it is?

I think it's an enemy Soldier's HE charge - check your objective wheel for a "disarm" objective.

It just happened to me while playing on Shipyard (Resistance). No barricade was up, so no HE charge could have been planted. It happened immediately after i buffed a teammate, which seems to sometimes happen.

And i should have been more clear, it's not really a beep at all. Sounds like a shuffle/shake.

I think it does that when your out of supplies... that's when it makes that sound for me.

This. It happens when your less then 1 full pip.

Yeah, i noticed that shortly after my most recent post to this thread. It seems to only be with the Engineer though. Wierd.

Probably because you are buffing everyones damage

Probably because Engineers run out of pips more often...

Actually, no, I've never noticed it outside of when I'm actively trying to buff someone as any other class.

Don't play Engineer often enough to say for sure though.

on a different note, what does the circling red circle represent? on occasion a red circle will spin around my reticule for a couple of seconds

That's your reload indicator.

Along with a similar non-spinning one around your ammo count, and the fact that your gun is visibly being reloaded.

I think he is talking about the sticky grenade indicator it's a red circle with a grenade symbol on it that spins around.

The spinning thing is a sticky grenade that Operatives can throw on you, it'll blow up after a while (not sure if they can detonate on command or not). If you get near a teammate they are able to take it off before it blows up. If it blows up near them you both die though, careful!

On a side note heavy body types can just barely survive a sticky grenade that was stuck to them.

Don't worry, we can't control when they go off - it's on a timer - and quite a long one too.

Pretty much right there why I find Sticky Bombs almost worthless. Lol
It'd be better if they stuck to a surface, like a wall, when you threw it at one.

They're great for panicking enemies who are lone-wolfing.

I had an enemy lone-wolf camping in a spot where only Lights could reach, and I was the only Light on my team - several times I went there (it would have been a great flanking route), he had a turret and buffed SMG and killed me.

He never came out of hiding until the turret started shooting, though, so I didn't have a direct line where I could attack him. Once I worked out where he was, though, I managed to rebound a Sticky into his corner, and just laughed as he freaked out and ran away. Then exploded in mid-air jumping down towards his teammates. I fragged his turret, then continued on my way - he never came back.

You are correct, it's the Sticky Bomb. And, only Engineers can disarm them off of you.

EDIT: @obliviondoll, you can stick the Sticky Bomb anywhere, even the floor. I had my lil bro playing with me on Shipyard and I saw one right on the floor and he LITERALLY walks over it and it stuck to him.

So, you can place them in chokepoints you know enemies are going to come from/are coming from, and wait for them to run through and get attached!