I've discovered, through my many, many play-throughs, that the effect of the noise the player generates at given times during the game is cumulative; meaning that on playthroughs where I ran a lot and used my loudest weapons often, it turned out that the Alien was up my ass more often--spawning nearby and lingering too long--even in later missions. However, on play-throughs where I took my time, moving slowly like an electric supermarket scooter and staying silent...like an electric supermarket scooter--I've noticed that the Alien does not spawn nearby as frequently and mostly has its blood tea parties alone, in the vent, where it should be. Obviously, the Alien will be alerted to the noise you're making when it's nearby or on the same level, but I was surprised to learn that the Alien remembers a noisy player and aggros on to her more quickly, even as the game progresses to different levels.

This is all anecdotal of course, mind you, but until proven otherwise, I stand by my own experience and highly recommend this one very effective strategy for survival: crawl. That's right, just. crawl. loser. Get on your hands and knees and slither around on your stupid belly like the worthless worm you truly are. There are many achievement guides that tell you how to avoid the death screen by mashing the "esc" key, but isn't it better to just not die all together? Winter, Spring, Summer, or Fall, all you have to do is crawl. Never walk and absolutely never run! Not even once, for the entire game! They'll be times when you want to walk or run just to move a little faster. Don't do it. Not ever. I don't care how many sirens, alarms, or klaxxons are blaring and slinging their bright orange strobes all over the walls...just bend the knee and crawl like the useless infant baby child you are inside.

Crawling has three main benefits: 1) you're silent 2) your're out of sight lines 3) you're alive

I've crawled the entire game before both on Nightmare and on Novice to the get the hidden ONE SHOT achievement. In both difficulty scenarios, crawling went swimmingly. You will still die, of course, so remember the aforementioned "esc" key strategy, BUT, you will NOT die nearly as much or often AND some of the more convoluted objectives in later missions will be easier to accomplish without the Alien in rimming distance of your anus.

F.A.Q.
Isn't crawling for the entire game too slow and annoying?
Yes
Doesn't crawling totally cheese the AI and ruin the experience of the game?
Yes
I started a new playthru where I crawl the entire time, it works so well, it feels like cheating, Is it cheating?
Yes
I already knew this! Why did you waste your meaningless time writing a completely inadequate guide?
Yes

By the way, you're welcome, for providing you this "hot" tip, seven years after anyone gave a crap.