As Mike asked me to post on this I've read through the topic and done some extra research on what I "thought" I knew already. On my travels I found a useful site on turbo surge/stall and this is just a cut and paste of what it says, save me rewritting it. There is also an example of his experience with surge and how he fixed it (think his is diesel but application is relevant):
Originally Posted by turbo website
The condition known as turbo surge or stall occurs under heavy load conditions where boost pressures are elevated. The root cause is generally an imbalance of pressures between the turbine side and the compressor side of the turbo. When the forces on the turbine are no longer strong enough to drive the compressor side of the turbo adequately, stall occurs. Often on Powerstrokes the stall will manifest itself as a surging and the boost pressure in the manifold will fluctuate as the vehicle surges and you can hear an audible cyclic sound which corresponds with the surge.
EXAMPLE
The truck used for this article is my own 2001, F250 Powerstroke, crew cab, long box 4x4. The truck has a 4" exhaust, aFe intake, gauges and Banks Big Hoss 120hp chip. I would experience turbo stall/surge when towing a 30' travel trailer weighing about 10,000# over the passes at highway speeds of 65-75mph. When the boost would reach 17-22psi the stall/surge would be very strong and the boost would oscillate 2-4psi while making a wooh-wooh sound. If the throttle were pressed further so the boost would climb to 23-28psi boost the surge would go away, but its hard to explain to the state patrol why you are pulling a trailer at 90mph up hill.
The following is some information about our experience with this problem, its causes and how to fix it. The fix itself was quite simple, we installed a Banks quick turbo and compressor wheel and a Big Head actuator. This resolved the problem completely. The following are some photos and things which were observed and changed while the truck was apart.
Performance with Banks Turbo
When the truck was driven with the Banks turbo housing, compressor wheel and Big Head actuator no apparent performance gain was recognized. In fact the turbo comes on slower in the low end than the stock unit and we experienced considerably more turbo whistle than the stock unit as well. Mid range and high end performance seems about the same as stock with the same top boost pressures experienced at about 28psi. Under load towing where severe surge was experienced, absolutely no surge with Banks.
Next we tried the Banks Turbo housing with Big Head actuator and the stock compressor wheel. This reduced the whistle and helped bring back some of the low end. However under heavy load conditions an audible surging sound is present, but the boost gauge does not fluctuate. We suspected that the sound was being caused by the waste gate trying to open so a ball was placed in the tube leading tot he actuator to plug it. With the ball in place a noticeable reduction of the sound previously heard under heavy load conditions was present. The plugging of the Big Head actuator did not completely stop the sound but reduced it by more than half.
From this experience it would appear that the majority of the turbo surge or stall experienced with modified trucks especially those using large horse power chips which produce high boost pressures is associated with the waste gate. It would seem that the exhaust pressure against the waste gate is at least a contributing factor in unseating the waste gate allowing it to flutter effecting the pressures in the turbo charger.
The combination of the Banks turbo housing, compressor wheel and big head actuator work together to resolve the surge because the turbo housing is larger and flows more reducing the back pressure behind the turbine. The compressor wheel reduces the amount of drag on the wheel reducing the load on the turbine. The Big Head actuator with its larger, stronger spring helps to keep the waste gate shut. Since this was initially done we have found most cases of turbo strall can be resloved by replacing the stock compressor wheel with the Banks wheel and either plugging the actuator line to the waste gate or replacing it with a Banks Big head actuator. It is probably best to use the Banks Big Head so that control for opening the waste gate is not dependant soley on exhaust back pressure.
So all the chatter is is basically the blades rapidly changing acceleration direction, ie accelerating up to accelerating down (decelerating) and vice versa.
Surge is basically the fact that the turbo isn't dampened so when it gets to the required airflow for that rpm it "over shoots" and feeds too much, this causes it to try and slow down to the correct level, and again "under shoots". If the rpm was raised to a level and held you would hear chatter and as time went by it would disappear. as in diagram below.
As you can see it tries to get to the correct level and as time goes by the "over" and "under" shoots get smaller.
As stu said tho, its understanding it thats one thing and explaining it in a way for people to understand thats the problem!