Yhyjyk

I often have to work in our server room with a colleague, doing things like racking new systems, replacing drives, diagnosing issues, etc. It's a super loud room with around 50 units going at once, so the door is soundproofed. We're supposed to keep the door closed when we're in the room to keep the noise down, but it's making me uncomfortable to be alone with him.

It's not that he's done anything creepy yet, but it's just that if someone wanted to attack me, this would be the perfect room to do so. There are no windows in the room and as far as I can tell from the damping, it'd be really hard to hear anyone inside yelling for help.

So far I've tried propping the door open without saying anything, which he's just shut and said "you know we're supposed to keep this closed right?", I've tried making up an excuse that it's too warm with the door closed, I've tried saying that I'm just more comfortable with the door open, but he keeps closing it.

How should I try to politely tell him I'm uncomfortable being alone with someone else in a closed-off room, in case of an assault?

You can't.

First, the problem you describe is 100% about the situation in which you're working, and 0% about the person with whom you're working.

So any approach which mentions a specific person (along the lines of "I'm not comfortable working with you in the server room") shifts the focus away from your actual problem, and will cause an implication that you have some fear of specific people. Since that's not the case it will only cause confusion in general, and hurt feelings in the people you mention (who, per your description, have done absolutely nothing to produce your discomfort).

This is a work environment problem, and the person to discuss it with is someone that has authority to grant or limit access to the server room. Your issue seems, to me, to be that if you are working in the server room you need to be alone, and be assured that no one else can enter while you're working there.

That's not something that a coworker, carrying out their own assigned tasks in the room, can address. A manager or supervisor might be able to help you with those, but you'll have to collaborate with them to find a solution that suits your needs.

If that's not possible, or you're not comfortable pursuing it, then it may be impossible for you to work in the server room and feel comfortable. The most direct solution to that would be that your work tasks are changed so that you don't need to do anything in the server room at all.

If you want to bring it up to them directly, I would focus on you don't like being in enclosed spaces. Adding him to your reasoning is not going to do anything productive and will likely backfire and cause HIM to make an HR complaint (as many here would recommend to get ahead of potential issues).

You could also bring it up to your boss/supervisor and simply tell them you get anxious around the person, though be truthful that they have done nothing that may explain why you react this way.

If all you have is your anxiety/gut feeling, I would definitely NOT bring this to HR at this point in time.

You are looking at the issue from the wrong side...

Working alone in a soundproof noisy room full of heavy equipment is extremely dangerous: the colleague with you is your safety belt, be grateful he's with you!

Other answers propose surveillance cameras that could help in case of an accident: these cameras would not be manned 24/7 so should a server drop you would be stuck until someone notices you are missing.

Moreover it is common practice not to work alone in dangerous places to increase safety (underwater workers are an example).

In my opinion you could completely avoid the need to talk with your colleague shifting your focus to an actual issue (safety on the workplace: did you see a rack tilt? Scary, if you are on the wrong side!) instead of trying to avoid an hypotetical assault in a server room from a coworker that gave you no reason to fear him.

We're supposed to keep the door closed when we're in the room to keep
the noise down.

There's nothing you can do when it comes to your co-worker. He's just following protocol. It's the system that's flawed.

It's perfectly normal to feel at unease here, and you should ask the right people to do something about it. You are entitled to a safe work environment (we all are, heck, people use helmets, gloves and steel-boots for a reason). Besides, it makes sense for the company to make sure they don't get themselves into a sticky situation.

You should take this issue up with management and ask them to proceed to get better protocols up and running. That may include a change in the open door policy or having proper security systems installed.

If the company is not willing to change their protocols then you either have to decide if you really want to work in that work environment or not.

As a last resorts you could talk to the co-worker one on one to help him understand your concerns, while making sure it has nothing to do with him as a person. This is a slippery slope, however, and I'd only use this as a last resort if it means the difference of you staying or leaving. Even then, you'd still be breaking the company policy.