Whilst in lockdown, I bought a cheap cello bow from eBay to experiment with getting different types of sounds from some of the things in my flat. The long sustained tones that come from moving the bow slowly back and forth over resonant objects can create some fascinating (and useful) results when captured with a pair of contact mics.
A few years back I found an old zither in a charity shop near my parents house. It is horribly detuned, and makes the perfect 'tension' sound design tool! I also had a cheap thumb piano laying around, which produced some great sounds bowing the tines. In addition to the zither and mbira, I also got some nice tones from a bicycle wheel, clothes drying rack, metal ruler and other household items.
Scroll down to listen to some recordings, and some examples of how they can be manipulated to create tense, unsettling textures and ambiences.