08 suburban 5.3l replaced engine 10k miles

Hi,
I noticed that my burb started ideling rough at stop. Now it's starting to howl. I haven't noticed any dramatic fuel economy changes. I started to watch the fuel economy info. I know it's not exact, but it can change from 8-73 mpg in v4 or v8 mode. Anyone know what issue,if any, does this start to lead too? I plan on taking it back where I had the engine replaced but wanted some more info before I do.
thanks