A mantra cry is often a rhythmic, monotone ahh ahh noise, though some babies can be much more vocal. If there are tears though it sounds more like a proper cry
PUPD is not recommended under the age of 4 months and even after that was only intended to be used as a last resort. In the early days, shh pat or a variation of it is far more effective when used in combination with appropriate A times. You may have to experiment to see how your LO prefers it if the traditional way is too stimulating. You can try stroking the cheek or forehead, resting a hand on the chest, just shh, just pat etc Have you ever tried it I your arms to get LO used to the technique before trying it in the crib?
Does your LO have any discomfort issues eg gas or reflux?