Mistral Combi boiler making steam engine noises!

Our last boiler failed 6 mths out of Warranty, due to all the 'experts' concerned failing to notice the constant drop in pressure etc was due to it having a large hole in it-!
Had a new Mistral Combi boiler installed about 2 years ago, which makes 'steam engine' noises after it's fired (which appear to diminish when the rads are fired up), despite having vents installed and bleeding it (and the radiators) on several occasions, each time getting a considerable amount of air out of it.
Did this again just yesterday, then this morning - back to the steam engine noises, and cannot find any air in the boiler (conveniently located next to out bedroom, so at least we don't need an alarm in the morning-!)
Our new service engineer has advised Fernox (who I'm contacting directly) but I was wondering what other Users thoughts were on this incredibly irritating problem? It'd be nice to have a lie-in just occasionally. Many thanks!

Clearly a cowboy install. Sealed systems don't need multiple vents to get rid of air. Regular air build up is a problem that needs solving itself. 5 give you 10 that your system was not cleaned when the boiler was changed and is now seriously corroded.
It will need cleaning; FAQ and other announcements in bold black at top of forum will give you details

Thanks very much for the advice.

The new plumber says that they did drain the system when they installed the new boiler, so is it possible it wasn't done thoroughly enough/at all, and that could be causing the problem?

Thanks again!

Draining the system compares to a proper flush as cleaning the windscreen to a full service.
Search the forum for flush and powerflush, and you will get a pretty good idea.

bengasman said:

Sealed systems don't need multiple vents to get rid of air.

Click to expand...

Well you're entitled to an opinion.