Also make sure the rubber gasket under the blade assembly is seated and the blade collar is snug; a loose gasket or collar can cause rattling. Check the rubber drive coupling on top of the motor and the blender’s feet - cracked or missing rubber there leads to noise and wobble and both are cheap to replace. If the blades look nicked or slightly bent swap that assembly and add a bit more liquid so the load is balanced before ramping up.
Honestly - Oh man and same boat here with our two-story place and the baby monitor signal acting up all the time and especially through those solid old walls we have. I ended up trying a bunch of spots to place the base unit, you know moving it around until I found a sweet spot that wasn't blocked by too much furniture or anything. Finally settled on the hallway upstairs so yeah helped a ton.
If you're dealing with interference,, maybe check if other devices in the house are on the same channel or something, switching that up fixed it for us but then we had to readjust everything else... yeah. Works great overall now though. Hang in there, you'll get it sorted.
