Let me start out by saying that I think it's really awesome you're working so hard to help your Grey and take good care of him after your grandmother's loss. That said, it's a good idea to remember that you don't actually KNOW what went on at your grandmother's house. You mentioned that she didn't handle him at all, but maybe (just one scenario of many possibilities) that alarm call was the only way he WAS handled--it is possible she would pick him up or give him extra attention when he did that which would have taught him to do it. Another idea: Your grandmother was older and probably didn't have the best hearing, so these loud calls and truck noises may have been going on for a long time and simply didn't bother her as much--or she might have been keeping him in another room. There are a million other scenarios for him doing the things he does, but the way to solve them definitely isn't to cover him up and hope you can make it go away quickly. You may solve these problems next week, or they may go on for many years.
Unfortunately, you can't put a time limit on fixing a bird's issues, so if you have to have silence in your home for your work and won't go ahead and use earplugs then a bird may not be a good pick for your home (though I realize that at this point, it's a little late to go any other direction).
Like was mentioned before, it's not the parrot's fault--a bird can only be what it is. They are very smart, but not smart enough to change what they learned over many years instantly for your needs. Your guy obviously has learned that what we consider poor behavior is good behavior, and that can take a LONG time to fix. You are doing a great job trying different things, seeking out advice, and looking into different ways of training, I just don't want you to get your hopes up too much that you can change him, because while most birds will eventually come around, some never do, and part of keeping birds who are poorly socialized is accepting that they may never be that perfect bird you are hoping for.
I hope this didn't come across as too downer, I just wanted to try and be realistic--lots of people take in badly behaved birds with the assurance that they will get back to being well socialized and then the bird loses a second home when that doesn't happen--I just want to make sure you're aware that this is sort of like taking in a rescue bird in that sense!