Liverpool Echo is quoting a passenger as saying the captain announced it was a bird strike.
https://www.liverpoolecho.co.uk/news...duled-16176605
Sue Candler, who was travelling with her husband, said:
"As we were taking off I could hear from vibrations coming from behind me but we couldn't feel them. I wondered what was going on and then a few minutes after take off the pilot said we would be landing at Manchester due to a bird getting caught in the engine. We landed at about 3.30pm and were moved to another aircraft to go on to Lanzarote.
"The pilot came out and spoke to us as we were waiting to get off and explained that a bird had got caught in the engine during take off and it was safer to land and have the plane checked over."
Sounds like a very reasonable 7700 squawk to me but then I'm old but not bold.