The radio squeals even when off.
It does get louder when accelerating.
The car has a HK system.
One suggestion was to replace the amp.
This didn't make any difference.
Now the only thing I can think of is to replace the alternator.
How do I know if the alternator is 120a or 150a?
Would it make a difference if I just replaced it with a 150a?
What should I check to make sure the alternator will work?

You sure it is the radio squealing?

Alternator function can be checked by checking the power output, the battery voltage at the battery posts, with the engine running. One source claims a reading of 12.6V to 14V. 12.6V is in my experience low given modern cars have lots more electronics to power. My experience over the years is closer to 14V is normal.

Now just because it may read good at or shortly after engine start after some drive time the voltage can fall. A tenth of a voltage or two is not necessarily cause for concern. But if it drops more and continues to drop that's a sign the alternator is bad.

Now an alternator going bad even being bad doesn't have to make any noise.

One that does can still be providing good electrical power but the noise comes from bearings if it has brushes the brushes.

Another source of noise is the belt. If the alternator is manifesting a bad bearing this can cause the belt to slip.

If the alternator is making the noise you should be able to -- being very careful! -- confirm this. A mechanic's stethoscope is handy to help one pinpoint the source of a noise.

You can remove the accessory drive belt and check all the accessory drives for excessive bearing play. The drive that has the most (any really) is probably the bad one. Also, if the belt is a serpentine belt check the edges. Both edges should be dull. If one (probably the inner one) is sharp this is a sign the belt is not tracking properly and the edge of the belt is making contact with the side of an accessory drive pulley. The serpentine belt should not make contact with the pulley in this way.

Also check the idler roller and tensioner roller bearings for signs of trouble. The alternator doesn't have to be the source of the noise. And while you are there check the tensioner tensions the belt properly.

If you determine you have to replace the alternator the one time I had to replace an alternator I just had the tech replace the original (and bad) alternator with an identical unit from the factory. Now it was a remanufactured unit but done by the factory and it came with a nice warranty. And it was not that much more than a refurbished one from some other alternator rebuilder which pretty much provided no warranty.

Unless the factory has changed the alternator specification I'd stick with whatever capacity of alternator the car originally came with. An exception to this is if you have installed some additional electrical equipment that might put too much a strain on the stock alternator.

But you first need to confirm the alternator needs replacing.

The radio start squealing as soon as I open the door.
I might be fighting more than one issue.
Sometimes I can hear every bump.
I will check the alternator output this evening.

Can't remember where I came across this but another person reported what he thought was a radio squeal and it turned out to be some interaction between his cell phone and the car.

Hearing every bump is not an alternator problem nor a radio problem unless what hear is concurrent with hitting a bump.

You need to check the alternator output but you also need to check for any bearing play at the alternator and any other accessory drive. And check the idler roller bearings for any play or excessive resistance to turning. The squeal may be from an accessory drive belt slipping.

I went out to the mini with the phone turned off.
open the door and the squealing started.
I am assuming that it is not the phone.
It must be an electrical feed or grounding.
Any idea where the ground for the radio might be?
Read on another forum that it’s under the passenger seat.
Before I pull the seat and carpet I would like to confirm the R55 radio ground is in the same place.

Unfortunately I do not have access to any MINI wiring diagrams so I can't tell you where the radio ground wire is.

Don't know if any electronics have been added but running electrical lines next to speaker wires can produce noise.

Update: The manual that I have shows the different grounding points. For the radio, it under the drivers door seal. Odd place.
This weekend or next, I will look into what needs to be removed to get to it.

Did you ever figure out what was causing the interference? I'm getting similar noise through my Harmon Kardin even with the radio off. I suspect alternator because it changes tone with RPM, but I'll find wiring diagrams in the Bentley manual in the morning.

I have not been able to locate the ground issue.
My next set is to get an alternator that has the build in resister.

Do all the alternators have the resistor for the radio?
2010 Mini Cooper clubman base.
I am also getting concerned that the radio just quits for a second or two.
The display goes blank.

Any ideas?

Yes, Sounds like a bad ground connection. I would also pull the head unit out and check the connection.

Has the car had any aftermarket stereo or accessories added?

Is it possible to run a new ground from the radio case to the body?
Has anyone done this?

I have run into other issue that seem to have a line of check the ground.
Now I will start working on each ground. The first part is trying to come up with a list of the grounds.
Just so I don't do one twice.

This will take a while and I will let you know what I find.