You should make another CPU temperature test but make airflow guide from (for example) cardboard which forces top exhaust fans to draw air only from above CPU instead of everywhere. One user reported 10C CPU temp drop with such arrangement in Cosmos so uncontrolled airflow isn't minor thing. Big low speed fans generate more harder to hear low frequency noise which penetrates walls and structures better. Low frequency noise (basically vibration of molecules) might make some objects to vibrate at their resonant frequency making it audible, also size of room etc could amplify it at certain position. (try positioning speaker at different distances from wall and listen behaviour of low frequencies) Isn't there general rule for these situations: Not if you can stretch cables enough!