Metal is one of those sounds that aggravate dogs (there's been studies of the impact of metal cages on rescue dogs), and unfortunately your boy may have noise sensitivity. My noise sensitive male sheltie also barks at the blinds and windows, and the curtains. It is such a pain and I do avoid opening things to avoid his barking. You can reduce the sensitivity, but you may find you'll have to repeat the procedure after a while if he lapses (I've been a bit slack with this so my boy has lapsed lately).
There's 3 methods I can think of to deal with this, and you may want to try them all. I'll just go into them briefly for now as I'm heading off to bed.
With rewarding behaviours, as you are doing now, you need to treat at the point before he reacts, and you should break the actions down. So at first that may be when you put your hand near the blind or window. Give him a treat and get him to do distracting things - like obedience commands such as sit, shake hands etc, or play a game. Try each step in short training sessions, and only progress to the next step when you know he won't react to what you are doing. The next step may be putting your hand on the window, the following step may be starting to open it, and then you may need to breakdown how much you open it. Btw - if the food isn't working you may find a squeaky toy gets his attention better.
Another option is flooding. To do this effectively you need to record the sound and play it over and over and over (you can record it on your phone and then upload to the computer and put it on a loop). When you play it back start with the sound very, very low (remember dogs have great hearing) and play it while he's eating his dinner or otherwise distracted. Over the course of a week or two very gradually increase the sound but you never want him to be distracted by the sound. The idea is he gets so used to hearing the sound that he isn't bothered by it. It's a good idea to accompany this with the rewarding techniques - some of the behaviours may be entrenched by the action and not just the sound.
The third option is controlled barking - that's when you let them bark at something, but only after they've done a wait and you've given them the release word to bark. I've discussed it a number of times on the forum if you want to do a search. It worked fantastic for me - stopped my dog barking at appliances completely, he just gave up after a while, I think because he'd learnt to control himself he found he didn't need to bark. Given the age of your dog though you may find the other techniques the best way to start.