Washing machine E03 error
I had a similar problem with a Siemens IQ500 (Error E18). A reason for that sound might be that water isn't reaching the drain pump, so it's speeding up, getting stopped and restarted every few seconds. But that's just a guess.
A few things could cause that draining error:

Clean the drain filter.

Check if the drain hose is bent. Washing machines sometimes move, so they can bend the drain hose until water cannot go through anymore.

Connect another hose to the drain and let some water flow through it to make sure the drain itself isn't cloaked.

The drain pump might be corrupted or blocked. Try turning it with your fingers (it's right behind the drain filter). Doubt that's it though since you can still hear it running.

The outflow of the washing drum or the pipe between washing drum and drain pump might be clogged - are you missing any socks lately? :) That'd explain the sound you're hearing - water cannot reach the drain pump. You'll have to remove the front cover to get there though. Once you've removed the pipe you should be able to see (or feel) if there's something blocking the outflow of the washing drum (also a 20 EUR WiFi endoscope camera can be very helpful for checking if there are things inside the washing drum). If there's a sock stuck in the outflow of the washing drum, you can use a piece of thin copper or steel wire to push it out (stick it through the little holes in the washing drum).


In my case the problem was caused by case 5. I pulled out 4 different things the washing machine has eaten over time.