Turbo chadder / knock?

Is it possible for a turbo to have a severe loud chadder that the owner said was a wastegate that was opening and closing at @ 1 to 2 times every second while under full throttle?  The engine and boost had no problem with smooth power delivery, it was just the noise.  This car has been sitting for weeks, if not months.  This was not a mitsubishi motor but I am just wondering if anyone has came across a problem like this before? 
    

Its not really a chatter if it was that..  Does it sound like it is metal hitting?    Because you would hear a wissle or a realeasing of air, which wouldnt really be a chatter..

It was a metal hitting metal sound.  The turbo whine was quite loud with no changing of the pitch, the hitting sound was a really quick tap against metal that was just a bit louder than the turbo whine.  The power increase was very linear with no jumps or peaks and no release of air noises.

metal on metal could be the head or block..  The turbo may have "death wine" but it wouldnt be making a metal on metal sound..   What kind of car is this on?

It's a 86 Grand National with the 3.8L turbo/ intercooler motor.  It has had a couple of things done to it. ( injectors, empty cat, exhaust, K&N, shift kit and a manual boost control )
  I was thinking that some of the guys on this forum has had more experience with turbos than the average car buff and was thinking that the (?) manual boost control might be the problem.  It was like someone was under the hood hitting 2 frying pans with a large spoon, while this was going on, it was pulling like mad.     
Thanks for you inputs