Sounds like the buzz (rattle) bolt to me.
I torqued those bolts when I replaced the top radiator hose. I removed the horn mounting bracket, though, so that's worth a re-visit, at least.
Well when mine started driving me crazy, I torqued it hard. It only decreased rattling a little bit, so I went back and torqued it as hard as I could physically pull (nut squealing on the threads). I think I was pulling/bending the frame members the bolt runs through together. The rattle is not the bolt, it is the internal spacer the bolt runs through, inside the frame members, that rattles. Some have got in there and added a washer inside the frame members and/or spooged RTV sealant inside the spacer.
I suppose it could be your timing belt tensioners or the radiator itself (top bolt?). When noises start right after maintenance, the noise is most often related to that maintenance.
My rattlebolt noise just started out of the blue, no precipitating factor I could ever discover. Irritating as hell though.