Anything that travels faster than the speed of sound yields a sonic boom. This includes the crack of a whip, for instance. Obviously that one is smaller than what a jet produces.

You can play some games with which direction the energy of the boom goes and build planes that tend to send their boom more upwards than downwards, but you can't eliminate it going in all directions.

I have a loooot of issues with "UFOs are indeed alien crafts!", but certainly one of the bigger ones is the sheer stack of absurdities required to believe them, if you put them all in a pile at once and look at them seriously, and think about what they would mean if true. It isn't just that they have engines capable of absurd accelerations. It's that they also seem to use them in ways that under other circumstances we'd call "lacking purpose", i.e., they're just cruising along in one direction at twice the speed of sound, then all of a sudden go "oh, crap! I'm going the wrong way!" and slam themselves 120 degrees in another direction in a fraction of a second. It's that they also require "inertial dampers" in addition to the pile of other technologies of dubious physical possibility. We have to believe in ships that don't just move in absurd ways but also control the air around them to not be the noisiest damned things on planet Earth. It's that we also have to believe these apparently advanced aliens tend to crash surprisingly often, and that they don't want to be seen but just sort of accidentally get seen a little bit. We also have to believe in aliens that are staggeringly more advanced than humans but don't just, you know, do whatever it is they want to do because they are indeed that much more advanced, but are dicking around for decades at a time, apparently engaging in "negotiations" with our government despite their being so much more advanced that they should have no particular need to "negotiate" about anything.

It's not just the nature of the claims themselves... it's that if you take the claims seriously, they imply further absurdities that really don't make a lot of sense. Even if you take the technology claims on faith, the whole "UFOs are aliens!" tend to imply an alien species that has no intelligent motivations. Any two or three of them maybe I could harmonize into a plausible theory, but the whole pile is insane.

("Aliens" also have this really interesting characteristic where they just happen to have capabilities awfully close to what someone in 1950 might have considered to be the apex of possible physical technology, but rather bizarrely don't much resemble what the apex of physical technology looks like in 2021. For instance, the whole "aliens are genetically experimenting on humanity!" is rather silly if you consider an alien race who has computing power on their smallest scout ship exceeding everything humanity has ever possessed. Why the hell would you run around "experimenting" on humans when you can just compute the answer in a few seconds? Why would you "experiment" on humanity when odds are you can simply build any life form you like directly, without even needing to base it on some existing life form? And why would you be dicking around with life at all when you can build AIs thousands of times better for some purpose than any amount of human experimentation will yield? The whole memeset is very suspiciously stuck in a very particular period of time, and not reacting to 70 years of technological progress at all. See also our hand-held drones; increasingly real tech is running beyond what these "aliens" seem to posses, if we ignore all the inertial control and such. Or our existing surveillance tech; why would they need to watch us with crash-prone enormous spacecraft when they could just loft a couple dozen satellites into orbit and watch us at will, centuries before humans could even understand what that means?)

If UFOs are aliens, we have as much evidence that they are dumb as a bag of bricks as we do that they are hyperadvanced superintelligences, honestly.