One of the big suppliers Atlas Copco (others are available) talks of typical noise values of 85dB(A) which is probably a brand new well maintained machine.
By comparison typical road noise is between 68dB(A) car up to @ 78dB(A) Heavy Goods.
I would speculate an older machine, even in peak maintance will be noisier than 85dB(A).
Whilst you may not be in the immediate vicinity of housing with the right weather conditions certain noises can travel for miles and at night be considered a statutory nuisance.
You talk of other units being nearby - if you rent your building you may wish to talk to the landlord as you may not have the rights to place the compressor outside and if you disturb others they will be getting the complaints from your neighbours.
Be aware that if you place the compressor between two buildings especially indutrial style units the noise could be significantly louder by cumulative effect as it bounces off the walls.
As to the tin shed fully agree this is not really a suitable "enclosure" being likely to only keep rain and snow off the unit.
Other issues to consider:
Ground condition - remember these things need lubricants which should not be permitted to permeate soil or enter ground water drainage. A thin skim of concrete is not suitable for vibrating plant to sit upon.
Temperature - most air compressors have water traps / vents which if outside uninsulated can freeze and possibly rupture. If it is a rotary vane style these are engineered to quite tight tolerances where the thermal expansion and contraction of being outside could reduce the working life / increase maintenance requirements.
Maintenance - is the proposed enclosure sufficiently sized to allow suitable personnel access all the way around the compressor.
Security - how do you stop unauthorised persons accessing & interfering with the compressor given you are on an open site.
There are of course many mitigations against noise and reverbaration:
A large concerete plinth.
Vibration damping floor mounts and pipe supports.
Insulated wall panels / lagging damping sheets for pipework.
A newer quieter compressor or multiple units servicing one or several tasks rather than the whole buiding.