Pull the trigger and water surges - Karcher 2500 PSI Pressure Washer

SOURCE: water surges when you pull the trigger

If you have not flushed all air out of hoses prior to starting motor this may be the first symptom. If the water pressure from the spigot is competing with the clothes washer, shower etc. there may not be enough pressure to supply the demand of your pressure washer. A gauge attached to the spigot can give you a gallons per minute reading or take a five gallon bucket and time how long it takes to fill the bucket. ie: if the bucket fills in 2 minutes you have 2.5 gallons per minute. You may need this much to get un-interupted pressure for a 2400- or 2500 unit. Most residential properties are pipe sized from 3/4" pipe at the water main spigot at the front of the house down to a 1/2" pipe size at the rear of the house to maintain pressure.

SOURCE: water surges when you pull the trigger

Not enough water feed would cause pump cavitation. It sounds awful.
It sounds like you either have an engine that is revving up and down which will cause the water to surge. This is most likely a dirty carburetor. You should notice a distinct increase and decrease of the engine noise.
Or you have one or more check valves that are not seating properly. In this case the engine noise will be pretty constant but the spray out the wand will pulsate.