Sounds like a Diesel engine after extensive work

Hi all,
I'm new here and have reading a lot of posts, but have yet to find a good answer to my specific problems. Forgive me if I missed something.
My 2002 forester just had a head gasket replacement (both heads), timing belt was replaced along with the other drive belts. The valves were adjusted and spark plugs replaced. $1500.00 later my car sounds like a diesel truck and seems to run a bit sluggish. I can't describe the noise any better than that- it sounds like an old diesel truck- when I first start it up, when it idles, when I accelerate. I do live in the mountains and the car definitely struggles to get up the hills more than before.
Before all of this work, my car was peppy and quiet. Because a lot of things have been done to the car, I need help figuring out what could have gone wrong.
All of the work was done at my local Subaru dealership who are trusted by many around here, although we don't have a lot of options. I plan to take it back to the dealership, but would like to get another opinion.

Any ideas? Advice?

Thanks in advance,
Sarah