There are three sounds that are similar sounding; you always have to look at the situational contect and the body language.
A sharp, strong R R R, often in reaction to a sudden noise or an action of yours that they don't like signifies displeasure. Their body and face (dirty look) will reflect that.
A more continuous rrrrrrrrr in conjunction with bum wiggling is mild dominance rumblestrutting, which both genders do; sows most often when in season outside of bonding.
A more gentle off and on rrr rrr rrr with a relaxed body during lap time is purring and means pleasure.