Loose nut/rubber thing.

[Alex H]

I doubt that this is related to the ABS problem; but, this morning when filling up the washer fluid I noticed that the circled nut was loose and I had to tighten it back up. I'm guessing it's quite an important nut and holding something together, so it shouldn't be loose?

(stole someone else's pic)

 

[Henry]

the nut itself shouldn't be loose but it's not a problem if the rubber covered disc can be moved independently

that's your suspension top mount by the way