Why is my floor thumping and vibrating?

The way the floor in my apartment thumps and vibrates is becoming a serious issue now. We're talking about very heavy vibrations which almost feel like a mild electrocution, and these pounding shock wave thumps, they feel like someone kicking the back of your chair. I feel the shock waves from this traveling up my body. And they are starting to physically affect my nerves now, I'm starting to get these twitches.

What is making this worse is I can not figure out what is causing this. I only have one neighbor right now in my 2 story, 4 apartment building section. Well, condominiums. My neighbor lives right beneath me, and I thought that this is being created by something he's turning on. But he says that he is not doing anything or turning anything on to create the thumps and vibrations. And he says he's not feeling anything either. A couple friends of mine though who have come over do feel the thumps and vibrations.

I've been trying to figure out what is causing this. It's not either my or his air conditioner because I checked that out and can hear his unit on from my balcony when it's on. And I've gotten the thumps and vibrations when neither my or his air conditioner is on, and have not gotten them when they're both on. They don't come in timing with the air conditioning, so that's ruled out

It can't be the plumbing because there are days at a time some days where the thumps and vibrations don't happen at all, and I know we've still showered and flushed toilets. And then there are days where it is continuously around the clock, so plumbing is ruled out.

What I want to know is 2 things; what the hell is causing these thumps and vibrations. And, who to call. I'm in a condominium so there's no maintenance here. And property management won't do anything about it. My neighbor told both me and them that he's not turning anything on to create this. They believed him completely. I'm not sure. I don't know, it may not be him because I haven't seen any certain evidence that it is.

But, something is causing this. So, do I call a plumber? Electrition? Who do I call about this? This is physically affecting me now like I said further up, and I don't know where else to turn. Any ideas?