2003 Windstar theft alarm goes of randomly

[ATP2010]

I just had my engine replaced in my '03 Windstar. Since I have had the car back from the shop, the alarm goes off randomly. I was told to take it to Ford but I haven’t had time nor do I feel it’s fair I pay for another repair when this didn’t happen before I dropped the van off. I do not want to take it back to the shop that did the engine replacement as my car came back with dents and paint missing and no one will fess up to doing the damage. Are there any suggestions such as what to look for with this? When the alarm goes off, it doesn’t sound the same either, it’s accompanied by a rattling sound. Thank you for any help you can give me!