Interjection or noun?

[Unregistered]

In the following sentence, is "Incoming!" an interjection or a noun?

Incoming! was the last thing I heard before the round went off.

I believe it's a noun and the subject of the verb "was". Am I correct or is it an interjection?

[Tdol]

I'd punctuate it deifferently:

'Incoming!' was the last thing I heard....

The original use of the word alone was an interjection http://www.usingenglish.com/glossary/interjection.html

However, here, is not not an interjection- it is indeed the subject of the verb 'was'.