A broken spider is usually very distinctive.
As stated above, take a look at your drum.
The spider has 4 or 3 arms, spaced either 120° evenly apart or 2 on either side about 60° apart to each other 120° between the pairs.
If you grip the basket at the front rim, pull towards you.
Try at 2 different positions at least.
If the drum moves towards you as a whole it should be fine.
If you can basically tilt the drum within the tub, "bending" it, the spider is broken.
If it is only at a medium speed it is less likely it is the spider.
That would get louder as it gets faster, usually.
I more so think either a damper is going or something got loose, like a wire or a hose.
Broken dampers make less of a problem at high speeds.
At very high speeds the vibrations created become very high force but really short travel.
Those high forces are far beyond the dampers capabilities and the drum becomes basically free floating.
If a damper goes the damping characteristics change.
At very high speeds the machine might sound different but not vibrate more.
At lower speeds however the drum can travel more freely and more so unevenly.
Thus vibrations get more excessive.
Especially in medium speeds that becomes apparent.
At lower speeds the out of balance detection still runs and would cancel spins with to large drum travel.
At medium speed however spins cleared by OOB detection can now still create higher vibrations.
At worst just wait it out.
If it is a damper next thing you'll recognise is longer cycle times due to longer balanceing routines.
Then some harder to balance loads will get spun at lower speeds more often.
Those will get more and more of a Problem.
Next even easy to balance loads will get spun less and less reliably.
Until the point the drum starts to sag and you see marks appearing on the door glass.
Replacing the dampers isn't terribly complicated nor expensive.
Best is to replace all dampers (should be 3) at once.
When one gets weak, the others Cary the load and wear out rather quickly.
If you replace one the other will go out soon after, taking the other with it again.
So you end up in a continuous cycle of replacing dampers.
Lay machine on its side.
Bottom should be accessible.
Dampers should just be held in with plastic pegs.
These have wings on one side that you have to compress with a pair of pliers and then slide out.
Can be a bit tricky.
A tip:
If you can get these plastic pins cheap as well just change them over as well with new ones.
Every now and then these get deformed over time.
They don't fit snugly anymore and can cause perfectly good dampers to rattle and fail prematurely (hammer on nail effect).
Should be a cheap part.
Mount new dampers, make sure the pins are all the way in.
Upright machine again.
Check drum that it looks in its correct place.
It can happen that a spring at the top of the machine gets unhooked.
Then you'd just unscrew the lid and hook it back in.
Doesn't happen all that often though.