Coin Gone Down The Detergent Hose

It's a bit like Russian Roulette. The chances are it won't do any serious damage but it certainly could. It will behave completely randomly once inside. It could easily just end up inside the sump hose or pump filter and never be heard of again. But if it gets tipped on its edge and the revolving drum hits it then it can cause serious damage to the plastic outer tub.

If it were me I would disconnect the machine and tip it back and forth and from side to side to see if I could hear it sliding around in the tub. It's possible to get a coin to drop into the hole at the bottom of the tub into the sump hose. Once there it shouldn't be able to cause any serious damage. If you've already used the washing machine since, it could already have got into the sump hose.

Check the pump filter in case it's there. If you aren't going to try and find it or get an engineer out you need to listen carefully to it for the next several washes and if you hear it clanging about stop using it and try to get it out by the method I suggest or call an engineer.

Here's a useful article - how to find something stuck in washing machine drum