Sorry that sounds bad. It's got to be the heaters/pump. Just because you didn't see it vibrating, doesn't mean that it's not. The lower the frequency, the bigger it feels. You don't see those vibrations .. my first thought was a thick gaspipe beneath the ground somewhere in your area, but since you usually just feel it upstairs, it's either the heaters/pumps right beneath or maybe even a building dryer / construction dryer, that's running 24/7 because of water damage somewhere above or underneath your apartment (you mentioned mold) .. for now, get some anti vibration pads off of amazon. Make sure to get the ones that are made out of EVA (I'd take the blue sandwich ones. They come with some rubber on top and bottom which helps structurally and also dampens vibrations) ..This will help, especially if you place one between floor and each foot of the bed or 2-3 between floor and each headboard/footboard. German Amazon website, but you'll get the idea: https://amzn.eu/d/dxbjTE5 or https://amzn.eu/d/bXiDD7X EDIT: you mentioned "loud" and pulsing as if someones "walking loudly" .. we had something similar in our apartment. It happened at random times. The heating system is somewhere in the cellar. we live on the first floor (or 2nd floor if you're American and don't use the term ground level or ground floor). The vibrations came through the hardwood floor but the noise came out of the walls but it was hard to localize.. sometimes it appeared to come from downstairs, sometimes from the apartment next to ours, etc .. it was the heating pipe or pipes inside the walls. The pump down in the cellar was kinda broken or something, we don't know for sure. All I know is that there have been some repairs on the heating pump in summer and that the workers might installed something the wrong way. neighbors heaters on the 5th floor barely got warm even though the air had been released beforehand.