Impending turbo failure -- how do you tell?

[biturbowagon]

I've recently had two turbos blow within about 150 miles of each other.

 

I didn't notice any problem until the engine lost power and I heard noises from under the hood.

 

In fact, the CEL never came on in either case, even as the engine self-destructed the second time.

 

Question: are there any indicators to look or listen for before the turbo blows?

 

For example, with the Audi 2.7tt engine, there is a characteristic "siren" sound that often precedes turbo failure.

 

I do not suspect that my third turbo is on its way out. I'm hoping that answers to this question will be academic for me and helpful for others. But you never know; ergo my question.

 

Thanks.