Howling A/C on Accent 2003

On service after 15000km they compensated 30 "lost" grams of freon, and
after that, my A/C is not leaking any more, but now it is howling, the
warmer weather, howling is louder. When it is turned on
and outer temperature is greater than 30 Celsius, it is wery annoying, and
A/C is turning on and off (compressor only) rapidly (2-3 sec. on, 2-3 sec.
off). Inner temperature is OK. Think that I have
pressure problem. I went to service on 30000km., told them about it, but
nothing is done, everything is OK according to them.