The Carrier Performance 15 Seer single stage is one of the quietest units on the market and Carrier's quietest unit. Nearly silent against typical outdoor background sound levels. I think York Affinity units are also super quiet. The Carrier compact side discharge is less efficient and noisier.
Make sure you get your units sized correctly. IT would need to be a HUGE townhouse to require 9 tons of cooling... even in Houston or South Florida, we're talking 4000 sqft+ in most cases. Where do you live?
To save some money, you can pair the 15 Seer Performance series AC with a Comfort Series Air handler with just a regular PSC motor. It will still get around 14 SEER and will be quiet.
If the deck is open, you could fit the units up against the fence as long as you meet hte minimum clearnace on the other 3 sides. But 3' of width should be enough as long as the other 3 sides are open. The clearance requirements you can find by searching for (insert model number from Carrier website and add "data sheets" to a google search.
Pick the most important factors in system selection, these 5 items tend to conflict with each other:
Small Physical Size