I generally recommend against covering A/C units. They are meant to sit outside year round. Covering them leads to moisture problems, rodents, nuisance service calls (when you start it up without removing the cover), etc. What model# is your unit? The sound blanket may help some, but if it's really unbearably noisy something may be wrong with it.
"We always fix it right the second time".
All posts are strictly my opinions and not those of my employer. I cannot make statements on behalf of my employer.
All advice posted by me is for educational purposes only, HVAC repairs should only be conducted by trained & qualified people.